z3c.form 1.2.0

An advanced form and widget framework for Zope 3

This package provides an implementation for HTML forms and widgets. The goal
is to provide a simple API but with the ability to easily customize any data or
steps.

Detailed Documentation
**********************

=================
Forms and Widgets
=================

This package provides an implementation for HTML forms and widgets. The goal
is to provide a simple API but with the ability to easily customize any data or
steps. This document, provides the content of this package's documentation
files. The documents are ordered in the way they should be read:

- ``form.txt`` [must read]

Describes the setup and usage of forms in the most common usages. Some
details are provided to the structure of form components.

- ``subform.txt`` [must read]

Introduces the complexities surrounding sub-forms and details two classes of
sub-forms, including code examples.

- ``field.txt`` [must read]

Provides a comprehensive explanation of the field manager API and how it is
to be used.

- ``button.txt`` [must read]

Provides a comprehensive explanation of the button manager API. It also
outlines how to create buttons within schemas and how buttons are converted
to actions.

- ``zcml.txt`` [must read]

Explains the ZCML directives defines by this package, which are designed to
make it easier to register new templates without writing Python code.

- ``validator.txt`` [advanced users]

Validators are used to validate converted form data. This document provides
a comprehensive overview of the API and how to use it effectively.

- ``widget.txt`` [advanced users]

Explains in detail the design goals surrounding widgets and widget managers
and how they were realized with the implemented API.

- ``action.txt`` [advanced users]

Explains in detail the design goals surrounding action managers and
actions. The execution of actions using action handlers is also covered. The
document demonstrates how actions can be created without the use of buttons.

- ``value.txt`` [informative]

The concept of attribute value adapters is introduced and fully
explained. Some motivation for this new and powerful pattern is given as
well.

- ``datamanager.txt`` [informative]

Data managers are resposnsible for accessing and writing the data. While
attribute access is the most common case, data managers can also manage
other data structures, such as dictionaries.

- ``converter.txt`` [informative]

Data converters convert data between internal and widget values and vice
versa.

- ``term.txt`` [informative]

Terms are wrappers around sources and vocabularies to provide a common
interface for choices in this package.

- ``util.txt`` [informative]

The ``util`` module provides several helper functions and classes. The
components not tested otherwise are explained in this file.


Browser Documentation
---------------------

There are several documentation files in the ``browser/`` sub-package. They
mainly document the basic widgets provided by the package.

- ``README.txt`` [advanced users]

This file contains a checklist, ensuring that all fields have a widget.

- ``<fieldname>.txt``

Each field name documentation file comprehensively explains the widget and
how it is ensured to work properly.

=====
Forms
=====

The purpose of this package, of course, is the development forms in a simple
as possible way and still providing all the hooks to do customization at any
level as required by our real-world use cases. Thus, once the system is setup
with all its default registrations, it should be trivial to develop a new
form.

The strategy of this document is to provide the most common, and thus
simplest, case first and then demonstrate the available customization
options. In order to not overwhelm you with our set of well-chosen defaults,
all the default component registrations have been made prior to doing those
examples:

>>> from z3c.form import testing
>>> testing.setupFormDefaults()

Before we can start writing forms, we must have the content to work with:

>>> import zope.interface
>>> import zope.schema
>>> class IPerson(zope.interface.Interface):
...
... name = zope.schema.TextLine(
... title=u'Name',
... required=True)
...
... gender = zope.schema.Choice(
... title=u'Gender',
... values=('male', 'female'),
... required=False)
...
... age = zope.schema.Int(
... title=u'Age',
... description=u"The person's age.",
... min=0,
... default=20,
... required=False)

>>> from zope.schema.fieldproperty import FieldProperty
>>> class Person(object):
... zope.interface.implements(IPerson)
... name = FieldProperty(IPerson['name'])
... gender = FieldProperty(IPerson['gender'])
... age = FieldProperty(IPerson['age'])
...
... def __init__(self, name, gender=None, age=None):
... self.name = name
... if gender:
... self.gender = gender
... if age:
... self.age = age
...
... def __repr__(self):
... return '<%s %r>' %(self.__class__.__name__, self.name)

Okay, that should suffice for now.

What's next? Well, first things first. Let's create an add form for the
person. Since practice showed that the ``IAdding`` interface is overkill for
most projects, the default add form of ``z3c.form`` requires you to define the
creation and adding mechanism.

__Note__:

If it is not done, ``NotImplementedError``'s are raised:

>>> from z3c.form.testing import TestRequest
>>> from z3c.form import form, field

>>> abstract = form.AddForm(None, TestRequest())

>>> abstract.create({})
Traceback (most recent call last):
...
NotImplementedError

>>> abstract.add(1)
Traceback (most recent call last):
...
NotImplementedError

>>> abstract.nextURL()
Traceback (most recent call last):
...
NotImplementedError


Thus let's now create a wroking add form:

>>> class PersonAddForm(form.AddForm):
...
... fields = field.Fields(IPerson)
...
... def create(self, data):
... return Person(**data)
...
... def add(self, object):
... self.context[object.name] = object
...
... def nextURL(self):
... return 'index.html'

This is as simple as it gets. We explicitely define the pieces that are
custom to every situation and let the default setup of the framework do the
rest. Yes, this looks extremely similar to ``zope.formlib`` and it is
intentional so, because we really like the simplicity of ``zope.formlib``'s
way of dealing with the common use cases.

During the test setup we also brought up a root folder already, so let's try
to add a new person object there. For this add form, of course, the context is
now the root folder:

>>> request = TestRequest()
>>> add = PersonAddForm(root, request)

Since forms are not necessarily pages -- in fact often they are not -- they
must not have a ``__call__`` method that does all the processing and rendering
at once. Instead, this form embraces the update/render pattern. Thus, we first
call the ``update()`` method.

>>> add.update()

Actually a lot of things happen during this stage. Let me step through it one
by one pointing out the effects.


Find a widget manager and update it
-----------------------------------

The default widget manager knows to look for the ``fields`` attribute in the
form, since it implements ``IFieldsForm``:

>>> from z3c.form import interfaces
>>> interfaces.IFieldsForm.providedBy(add)
True

The widget manager is then stored in the ``widgets`` attribute as promised by
the ``IForm`` interface:

>>> add.widgets
<z3c.form.field.FieldWidgets object at ...>

The widget manager will have three widgets, one for each field:

>>> add.widgets.keys()
['name', 'gender', 'age']

When the widget manager updates itself, several sub-tasks are processed. The
manager goes through each field, trying to create a fully representative
widget for the field.

Field Availability
~~~~~~~~~~~~~~~~~~

Just because a field is requested in the field manager, does not mean that a
widget has to be created for the field. There are cases when a field
declaration might be ignored. The following reasons come to mind:

* No widget is created, if the data is not accessible in the content.
* A custom widget manager has been registered to particularly ignore a field.

In this simple case, all fields will be converted to widgets.

Widget Creation
~~~~~~~~~~~~~~~

So first it instantiates the widget with the field. During the process,
several pieces of information are transferred from the field to the widget:

>>> age = add.widgets['age']

# field.title -> age.label
>>> age.label
u'Age'

# field.required -> age.required
>>> age.required
False

All these values can be overridden at later stages of the updating
process.

Widget Value
~~~~~~~~~~~~

The next step is to determine the the value that should be displayed by the
widget. There are three places where the value could come from:

* The field's default value.
* The request, in case a form has not been submitted or an error occurred.
* The context of the form, which represents the displayed content.

Since we are currently building an add form, the only the first two places are
effective. And since we do not have anything in the request, the value should
be the field's default value or be empty.

>>> age.value
u'20'

Since the widget only deals with output-ready values, the system also converts
the value using a data converter.

Widget Mode
~~~~~~~~~~~

Now the widget manager looks at the field to determine the widget mode -- in
other words whether the widget is a display or edit widget. In this case all
fields are input fields:

>>> age.mode
'input'

Deciding which mode to use, however, might not be a trivial operation. It
might depend on several factors:

* The permission to the content's data value
* The manual ``mode`` flag in the field
* The ``read_only`` flag in the schema field


Widget Attribute Values
~~~~~~~~~~~~~~~~~~~~~~~

As mentioned before, several widget attributes are optionally overridden when
the widget updates itself:

* label
* hint
* required
* mode

Since we have no customization components registered, all of those fields will
remain as set before.


Find an action manager, update and execute it
---------------------------------------------

After all widgets have been setup correctly, the actions are setup. By
default, the form machinery uses the button declaration on the form to create
its actions. For the add form, an add button is defined by default, so that we
did not need to create our own. Thus, there should be one action:

>>> len(add.actions)
1

The add button is an action and a widget at the same time:

>>> addAction = add.actions['add']
>>> addAction.title
u'Add'
>>> addAction.value
u'Add'

After everything is setup, all pressed buttons are executed. Once a submitted
action is detected, a special action handler adapter is used to determine the
actions to take. Since the add button has not been pressed yet, no action
occurred.


Rendering the form
------------------

Once the update is complete we can render the form. Since we have not
specified a template yet, we have to do this now. We have prepared a small and
very simple template as part of this test:

>>> import os
>>> from zope.app.pagetemplate import viewpagetemplatefile
>>> from z3c.form import tests
>>> def addTemplate(form):
... form.template = viewpagetemplatefile.BoundPageTemplate(
... viewpagetemplatefile.ViewPageTemplateFile(
... 'simple_edit.pt', os.path.dirname(tests.__file__)), form)
>>> addTemplate(add)

Let's now render the page:

>>> print add.render()
<html>
<body>
<form action=".">
<div class="row">
<label for="form.widgets.name">Name</label>
<input type="text" id="form.widgets.name" name="form.widgets.name"
class="textWidget" value="" />
</div>
<div class="row">
<label for="form.widgets.gender">Gender</label>
<select id="form.widgets.gender" name="form.widgets.gender:list"
class="selectWidget" size="1">
<option id="form.widgets.gender.novalue"
value="--NOVALUE--">no value</option>
<option id="form.widgets.gender.0" value="male">male</option>
<option id="form.widgets.gender.1" value="female">female</option>
</select>
<input name="form.widgets.gender-empty-marker" type="hidden"
value="1" />
</div>
<div class="row">
<label for="form.widgets.age">Age</label>
<input type="text" id="form.widgets.age" name="form.widgets.age"
class="textWidget" value="20" />
</div>
<div class="action">
<input type="submit" id="form.buttons.add" name="form.buttons.add"
class="submitWidget" value="Add" />
</div>
</form>
</body>
</html>


Submitting an add form successfully
-----------------------------------

Let's now fill the request with all the right values so that upon submitting
the form with the "Add" button, the person should be added to the root folder:

>>> request = TestRequest(form={
... 'form.widgets.name': u'Stephan Richter',
... 'form.widgets.gender': ['male'],
... 'form.widgets.age': u'20',
... 'form.buttons.add': u'Add'}
... )

>>> add = PersonAddForm(root, request)
>>> add.update()

>>> sorted(root)
[u'Stephan Richter']
>>> stephan = root[u'Stephan Richter']
>>> stephan.name
u'Stephan Richter'
>>> stephan.gender
'male'
>>> stephan.age
20


Submitting an add form with invalid data
----------------------------------------

Next we try to submit the add form with the required name missing. Thus, the
add form should not complete with the addition, but return with the add form
pointing out the error.

>>> request = TestRequest(form={
... 'form.widgets.gender': ['male'],
... 'form.widgets.age': u'23',
... 'form.buttons.add': u'Add'}
... )

>>> add = PersonAddForm(root, request)
>>> add.update()

The widget manager and the widget causing the error should have an error
message:

>>> [error for error in add.widgets.errors]
[<ErrorViewSnippet for RequiredMissing>]

>>> add.widgets['name'].error
<ErrorViewSnippet for RequiredMissing>

Let's now render the form:

>>> from z3c.form import testing
>>> add.template = viewpagetemplatefile.BoundPageTemplate(
... viewpagetemplatefile.ViewPageTemplateFile(
... testing.getPath('../tests/simple_edit.pt'), ''), add)
>>> print add.render()
<html>
<body>
<i>There were some errors.</i>
<ul>
<li>
Name: <div class="error">Required input is missing.</div>
</li>
</ul>
<form action=".">
<div class="row">
<b><div class="error">Required input is missing.</div>
</b><label for="form.widgets.name">Name</label>
<input type="text" id="form.widgets.name" name="form.widgets.name"
class="textWidget" value="" />
</div>
<div class="row">
<label for="form.widgets.gender">Gender</label>
<select id="form.widgets.gender" name="form.widgets.gender:list"
class="selectWidget" size="1">
<option id="form.widgets.gender.novalue"
value="--NOVALUE--">no value</option>
<option id="form.widgets.gender.0" value="male"
selected="selected">male</option>
<option id="form.widgets.gender.1" value="female">female</option>
</select>
<input name="form.widgets.gender-empty-marker" type="hidden"
value="1" />
</div>
<div class="row">
<label for="form.widgets.age">Age</label>
<input type="text" id="form.widgets.age" name="form.widgets.age"
class="textWidget" value="23" />
</div>
<div class="action">
<input type="submit" id="form.buttons.add" name="form.buttons.add"
class="submitWidget" value="Add" />
</div>
</form>
</body>
</html>

Note that the values of the field are now extracted from the request.

Let's now also provide a negative age, which is not possible either:

>>> request = TestRequest(form={
... 'form.widgets.gender': ['male'],
... 'form.widgets.age': u'-5',
... 'form.buttons.add': u'Add'}
... )

>>> add = PersonAddForm(root, request)
>>> add.update()

>>> [(view.widget.label, view) for view in add.widgets.errors]
[(u'Name', <ErrorViewSnippet for RequiredMissing>),
(u'Age', <ErrorViewSnippet for TooSmall>)]

But the errror message for a negative age is too generic:

>>> print add.widgets['age'].error.render()
<div class="error">Value is too small</div>

It would be better to say that negative values are disallowed. So let's
register a new error view snippet for the ``TooSmall`` error:

>>> from z3c.form import error

>>> class TooSmallView(error.ErrorViewSnippet):
... zope.component.adapts(
... zope.schema.interfaces.TooSmall, None, None, None, None, None)
...
... def update(self):
... super(TooSmallView, self).update()
... if self.field.min == 0:
... self.message = u'The value cannot be a negative number.'

>>> zope.component.provideAdapter(TooSmallView)

>>> add = PersonAddForm(root, request)
>>> add.update()
>>> print add.widgets['age'].error.render()
<div class="error">The value cannot be a negative number.</div>


Additional Form Attributes and API
----------------------------------

Since we are talking about HTML forms here, add and edit forms support all
relevant FORM element attributes as attributes on the class.

>>> add.method
'post'
>>> add.enctype
'multipart/form-data'
>>> add.acceptCharset
>>> add.accept

The ``action`` attribute is computed. By default it is the current URL:

>>> add.action
'http://127.0.0.1'

The name is also computed. By default it takes the prefix and removes any
trailing ".".

>>> add.name
'form'

The template can then use those attributes, if it likes to. Since we are
talking about templates, in this example we set the template manual, but if no
template is specified, the system tries to find an adapter. Initially, there
is no adapter, so rendering the form fails:

>>> add.template = None
>>> add.render()
Traceback (most recent call last):
...
ComponentLookupError: ((...), <InterfaceClass ...IPageTemplate>, u'')

The form module provides a simple component to create template factories:

>>> factory = form.FormTemplateFactory(
... testing.getPath('../tests/simple_edit.pt'), form=PersonAddForm)
>>> zope.component.provideAdapter(factory)

Now the factory will be used to provide a template:

>>> print add.render()
<html>
...
</html>

Since a form can also be used as a page itself, it is callable:

>>> print add()
<html>
...
</html>


Changing Widget Attribute Values
--------------------------------

It frequently happens that a customer comes along and wants to slightly or
totally change some of the text shown in forms or make optional fields
required. Until now this always meant to implement a custom schema for this
client. With the z3c.form framework all attributes -- for which it is sensible
to replace a value without touching the code -- are customizable via an
attribute value adapter.

So let's change the label of the name widget from "Name" to "Full Name":

>>> from z3c.form import widget
>>> NameLabel = widget.StaticWidgetAttribute(
... u'Full Name', field=IPerson['name'])
>>> zope.component.provideAdapter(NameLabel, name='label')

When the form renders, the label should be changed:

>>> add = PersonAddForm(root, TestRequest())
>>> addTemplate(add)
>>> add.update()
>>> print add.render()
<html>
...
<div class="row">
<label for="form.widgets.name">Full Name</label>
<input type="text" id="form.widgets.name" name="form.widgets.name"
class="textWidget" value="" />
</div>
...


Adding a "Cancel" button
------------------------

Let's say a client requests that all add forms should have a "Cancel"
button. When the button is pressed, the user is forwarded to the next URL of
the add form. As always, the goal is to not touch the core implementation of
the code, but make those changes externally.

Adding a button/action is a little bit more involved than changing a value,
because you have to isnert the additional action and customize the action
handler. Based on your needs of flexibility, multiple approaches could be
chosen. Here we demonstrate the simplest one.

The first step is to create a custom action manager that always inserts a
cancel action:

>>> from z3c.form import button
>>> class AddActions(button.ButtonActions):
... zope.component.adapts(
... interfaces.IAddForm,
... zope.interface.Interface,
... zope.interface.Interface)
...
... def update(self):
... self.form.buttons = button.Buttons(
... self.form.buttons,
... button.Button('cancel', u'Cancel'))
... super(AddActions, self).update()

After registering the new action manager,

>>> zope.component.provideAdapter(AddActions)

the add form should display a cancel button:

>>> add.update()
>>> print add.render()
<html>
...
<div class="action">
<input type="submit" id="form.buttons.add" name="form.buttons.add"
class="submitWidget" value="Add" />
</div>
<div class="action">
<input type="submit" id="form.buttons.cancel" name="form.buttons.cancel"
class="submitWidget" value="Cancel" />
</div>
...

But showing the button does not mean it does anything. So we also need a
custom action handler to handle the cancel action:

>>> class AddActionHandler(button.ButtonActionHandler):
... zope.component.adapts(
... interfaces.IAddForm,
... zope.interface.Interface,
... zope.interface.Interface,
... button.ButtonAction)
...
... def __call__(self):
... if self.action.name == 'form.buttons.cancel':
... self.form._finishedAdd = True
... return
... super(AddActionHandler, self).__call__()

After registering the action handler,

>>> zope.component.provideAdapter(AddActionHandler)

we can press the cancel button and we will be forwarded:

>>> request = TestRequest(form={'form.buttons.cancel': u'Cancel'})

>>> add = PersonAddForm(root, request)
>>> addTemplate(add)
>>> add.update()
>>> add.render()
''

>>> request.response.getStatus()
302
>>> request.response.getHeader('Location')
'index.html'

Eventually, we might have action managers and handlers that are much more
powerful and some of the manual labor in this example would become
unnecessary.


Creating an Edit Form
---------------------

Now that we have exhaustively covered the customization possibilities of add
forms, let's create an edit form. Edit forms are even simpler than add forms,
since all actions are completely automatic:

>>> class PersonEditForm(form.EditForm):
...
... fields = field.Fields(IPerson)

We can use the created person from the successful addition above.

>>> edit = PersonEditForm(root[u'Stephan Richter'], TestRequest())

After adding a template, we can look at the form:

>>> addTemplate(edit)
>>> edit.update()
>>> print edit.render()
<html>
<body>
<form action=".">
<div class="row">
<label for="form.widgets.name">Full Name</label>
<input type="text" id="form.widgets.name" name="form.widgets.name"
class="textWidget" value="Stephan Richter" />
</div>
<div class="row">
<label for="form.widgets.gender">Gender</label>
<select id="form.widgets.gender" name="form.widgets.gender:list"
class="selectWidget" size="1">
<option id="form.widgets.gender.novalue"
value="--NOVALUE--">no value</option>
<option id="form.widgets.gender.0" value="male"
selected="selected">male</option>
<option id="form.widgets.gender.1" value="female">female</option>
</select>
<input name="form.widgets.gender-empty-marker" type="hidden"
value="1" />
</div>
<div class="row">
<label for="form.widgets.age">Age</label>
<input type="text" id="form.widgets.age" name="form.widgets.age"
class="textWidget" value="20" />
</div>
<div class="action">
<input type="submit" id="form.buttons.apply" name="form.buttons.apply"
class="submitWidget" value="Apply" />
</div>
</form>
</body>
</html>

As you can see, the data is being pulled in from the context for the edit
form. Next we will look at the behavior when submitting the form.


Failure Upon Submission of Edit Form
------------------------------------

Let's now submit the form having some invalid data.

>>> request = TestRequest(form={
... 'form.widgets.name': u'Claudia Richter',
... 'form.widgets.gender': ['female'],
... 'form.widgets.age': u'-1',
... 'form.buttons.apply': u'Apply'}
... )

>>> edit = PersonEditForm(root[u'Stephan Richter'], request)
>>> addTemplate(edit)
>>> edit.update()
>>> print edit.render()
<html>
<body>
<i>There were some errors.</i>
<ul>
<li>
Age: <div class="error">The value cannot be a negative number.</div>
</li>
</ul>
<form action=".">
<div class="row">
<label for="form.widgets.name">Full Name</label>
<input type="text" id="form.widgets.name" name="form.widgets.name"
class="textWidget" value="Claudia Richter" />
</div>
<div class="row">
<label for="form.widgets.gender">Gender</label>
<select id="form.widgets.gender" name="form.widgets.gender:list"
class="selectWidget" size="1">
<option id="form.widgets.gender.novalue"
value="--NOVALUE--">no value</option>
<option id="form.widgets.gender.0" value="male">male</option>
<option id="form.widgets.gender.1" value="female"
selected="selected">female</option>
</select>
<input name="form.widgets.gender-empty-marker" type="hidden"
value="1" />
</div>
<div class="row">
<b><div class="error">The value cannot be a negative number.</div>
</b><label for="form.widgets.age">Age</label>
<input type="text" id="form.widgets.age" name="form.widgets.age"
class="textWidget" value="-1" />
</div>
<div class="action">
<input type="submit" id="form.buttons.apply" name="form.buttons.apply"
class="submitWidget" value="Apply" />
</div>
</form>
</body>
</html>


Successfully Editing Content
----------------------------

Let's now resubmit the form with valid data, so the data should be updated.

>>> request = TestRequest(form={
... 'form.widgets.name': u'Claudia Richter',
... 'form.widgets.gender': ['female'],
... 'form.widgets.age': u'27',
... 'form.buttons.apply': u'Apply'}
... )

>>> edit = PersonEditForm(root[u'Stephan Richter'], request)
>>> addTemplate(edit)
>>> edit.update()
>>> print edit.render()
<html>
...
<i>Data successfully updated.</i>
...

>>> stephan = root[u'Stephan Richter']
>>> stephan.name
u'Claudia Richter'
>>> stephan.gender
'female'
>>> stephan.age
27


Successful Action with No Changes
---------------------------------

When submitting the form without any changes, the form will tell you so.

>>> request = TestRequest(form={
... 'form.widgets.name': u'Claudia Richter',
... 'form.widgets.gender': ['female'],
... 'form.widgets.age': u'27',
... 'form.buttons.apply': u'Apply'}
... )

>>> edit = PersonEditForm(root[u'Stephan Richter'], request)
>>> addTemplate(edit)
>>> edit.update()
>>> print edit.render()
<html>
...
<i>No changes were applied.</i>
...


Changing Status Messages
------------------------

Depending on the project, it is often desirable to change the status messages
to fit the application. In ``zope.formlib`` this was hard to do, since the
messages were burried within fairly complex methods that one did not want to
touch. In this package all those messages are exposed as form attributes.

There are three messages for the edit form:

* ``formErrorsMessage`` -- Indicates that there was an error occurred while
applying the changes. This message is also available for the add form.

* ``successMessage`` -- The form data was successfully applied.

* ``noChangesMessage`` -- No changes were found in the form data.

Let's now change the ``noChangesMessage``:

>>> edit.noChangesMessage = u'No changes were detected in the form data.'
>>> edit.update()
>>> print edit.render()
<html>
...
<i>No changes were detected in the form data.</i>
...

When even more flexibility is required within a project, one could also
implement these messages as properties looking up an attribute value. However,
we have found this to be a rare case.


Creating Edit Forms for Dictionaries
------------------------------------

Sometimes it is not desirable to edit a class instance that implements the
fields, but other types of object. A good example is the need to modify a
simple dictionaries, where the field names are the keys. To do that, a special
data manager for dictionaries is available:

>>> from z3c.form import datamanager
>>> zope.component.provideAdapter(datamanager.DictionaryField)

The only step the developer has to complete is to re-implement the form's
``getContent()`` method to return the dictionary:

>>> personDict = {'name': u'Roger Ineichen'}
>>> class PersonDictEditForm(PersonEditForm):
... def getContent(self):
... return personDict

We can now use the form as usual:

>>> edit = PersonDictEditForm(None, TestRequest())
>>> addTemplate(edit)
>>> edit.update()
>>> print edit.render()
<html>
<body>
<form action=".">
<div class="row">
<label for="form.widgets.name">Full Name</label>
<input type="text" id="form.widgets.name"
name="form.widgets.name" class="textWidget"
value="Roger Ineichen" />
</div>
<div class="row">
<label for="form.widgets.gender">Gender</label>
<select id="form.widgets.gender" name="form.widgets.gender:list"
class="selectWidget" size="1">
<option id="form.widgets.gender.novalue"
value="--NOVALUE--">no value</option>
<option id="form.widgets.gender.0" value="male">male</option>
<option id="form.widgets.gender.1" value="female">female</option>
</select>
<input name="form.widgets.gender-empty-marker" type="hidden"
value="1" />
</div>
<div class="row">
<label for="form.widgets.age">Age</label>
<input type="text" id="form.widgets.age"
name="form.widgets.age" class="textWidget" value="20" />
</div>
<div class="action">
<input type="submit" id="form.buttons.apply"
name="form.buttons.apply" class="submitWidget"
value="Apply" />
</div>
</form>
</body>
</html>

Note that the name displayed in the form is identical to the one in the
dictionary. Let's now submit a form to ensure that the data is also written to
the dictionary:

>>> request = TestRequest(form={
... 'form.widgets.name': u'Jesse Ineichen',
... 'form.widgets.gender': ['male'],
... 'form.widgets.age': u'5',
... 'form.buttons.apply': u'Apply'}
... )
>>> edit = PersonDictEditForm(None, request)
>>> edit.update()

>>> from zope.testing.doctestunit import pprint
>>> pprint(personDict)
{'age': 5,
'gender': 'male',
'name': u'Jesse Ineichen'}


Creating a Display Form
-----------------------

Creating a display form is simple; just instantiate, update and render it:

>>> class PersonDisplayForm(form.DisplayForm):
... fields = field.Fields(IPerson)
... template = viewpagetemplatefile.ViewPageTemplateFile(
... 'simple_display.pt', os.path.dirname(tests.__file__))

>>> display = PersonDisplayForm(stephan, TestRequest())
>>> display.update()
>>> print display.render()
<html>
<body>
<div class="row">
Claudia Richter
</div>
<div class="row">
female
</div>
<div class="row">
27
</div>
</body>
</html>


Extending Forms
---------------

One very common use case is to extend forms. For example, you would like to
use the edit form and its defined "Apply" button, but add another button
yourself. Unfortunately, just inheriting the form is not enough, because the
new button and handler declarations will override the inherited ones. Let me
demonstrate the problem:

>>> class BaseForm(form.Form):
... fields = field.Fields(IPerson).select('name')
...
... @button.buttonAndHandler(u'Apply')
... def handleApply(self, form):
... print 'success'

>>> BaseForm.fields.keys()
['name']
>>> BaseForm.buttons.keys()
['apply']
>>> BaseForm.handlers
<Handlers [<Handler for <Button 'apply' u'Apply'>>]>

Let's now derive a form from the base form:

>>> class DerivedForm(BaseForm):
... fields = field.Fields(IPerson).select('gender')
...
... @button.buttonAndHandler(u'Cancel')
... def handleCancel(self, form):
... print 'cancel'

>>> DerivedForm.fields.keys()
['gender']
>>> DerivedForm.buttons.keys()
['cancel']
>>> DerivedForm.handlers
<Handlers [<Handler for <Button 'cancel' u'Cancel'>>]>

The obvious method to "inherit" the base form's information os to copy it
over:

>>> class DerivedForm(BaseForm):
... fields = BaseForm.fields.copy()
... buttons = BaseForm.buttons.copy()
... handlers = BaseForm.handlers.copy()
...
... fields += field.Fields(IPerson).select('gender')
...
... @button.buttonAndHandler(u'Cancel')
... def handleCancel(self, form):
... print 'cancel'

>>> DerivedForm.fields.keys()
['name', 'gender']
>>> DerivedForm.buttons.keys()
['apply', 'cancel']
>>> DerivedForm.handlers
<Handlers
[<Handler for <Button 'apply' u'Apply'>>,
<Handler for <Button 'cancel' u'Cancel'>>]>

But this is pretty clumsy. Instead, the ``form`` module provides a helper
method that will do the extending for you:

>>> class DerivedForm(BaseForm):
... form.extends(BaseForm)
...
... fields += field.Fields(IPerson).select('gender')
...
... @button.buttonAndHandler(u'Cancel')
... def handleCancel(self, form):
... print 'cancel'

>>> DerivedForm.fields.keys()
['name', 'gender']
>>> DerivedForm.buttons.keys()
['apply', 'cancel']
>>> DerivedForm.handlers
<Handlers
[<Handler for <Button 'apply' u'Apply'>>,
<Handler for <Button 'cancel' u'Cancel'>>]>

If you, for example do not want to extend the buttons and handlers, you can
turn that off:

>>> class DerivedForm(BaseForm):
... form.extends(BaseForm, ignoreButtons=True, ignoreHandlers=True)
...
... fields += field.Fields(IPerson).select('gender')
...
... @button.buttonAndHandler(u'Cancel')
... def handleCancel(self, form):
... print 'cancel'

>>> DerivedForm.fields.keys()
['name', 'gender']
>>> DerivedForm.buttons.keys()
['cancel']
>>> DerivedForm.handlers
<Handlers [<Handler for <Button 'cancel' u'Cancel'>>]>


Custom widget factories
-----------------------

Another important part of a form is, that we can use custom widgets. We can do
this in a form by defining a widget factory for a field. We can get the field
by get them from the fields collection e.g. fields['foo']. Which means, we can
define new widget factories by defining fields['foo'].widgetFactory = MyWidget.
Let's show a sample and define a custom widget:

>>> from z3c.form.browser import text
>>> class MyWidget(text.TextWidget):
... """My new widget."""
... css = u'MyCSS'

No we can define a field widget factory:

>>> def MyFieldWidget(field, request):
... """IFieldWidget factory for MyWidget."""
... return widget.FieldWidget(field, MyWidget(request))

We register the MyWidget in a form like:

>>> class MyEditForm(form.EditForm):
...
... fields = field.Fields(IPerson)
... fields['name'].widgetFactory = MyFieldWidget

We can see, that the custom widget get used in the rendered form:

>>> myEdit = MyEditForm(root[u'Stephan Richter'], TestRequest())
>>> addTemplate(myEdit)
>>> myEdit.update()
>>> print myEdit.render()
...
<html...
<input type="text" id="form.widgets.name"
name="form.widgets.name" class="MyCSS"
value="Claudia Richter" />
...

=========
Sub-Forms
=========

Traditionally, the Zope community talks about sub-forms in a generic manner
without defining their purpose, restrictions and assumptions. When we
initially talked about sub-forms for this package, we quickly noticed that
there are several classes of sub-forms with different goals.

Of course, we need to setup our defaults for this demonstration as well:

>>> from z3c.form import testing
>>> testing.setupFormDefaults()


Class I: The form within the form
---------------------------------

This class of sub-forms provides a complete form within a form, including its
own actions. When an action of the sub-form is submitted, the parent form
usually does not interact with that action at all. The same is true for the
reverse; when an action of the parent form is submitted, the sub-form does not
react.

A classic example for this type of sub-form is uploading an image. The subform
allows uploading a file and once the file is uploaded the image is shown as
well as a "Delete"/"Clear" button. The sub-form will store the image in the
session and when the main form is submitted it looks in the session for the
image.

This scenario was well supported in ``zope.formlib`` and also does not require
special support in ``z3c.form``. Let me show you, how this can be done.

In this example, we would like to describe a car and its owner:

>>> import zope.interface
>>> import zope.schema

>>> class IOwner(zope.interface.Interface):
... name = zope.schema.TextLine(title=u'Name')
... license = zope.schema.TextLine(title=u'License')

>>> class ICar(zope.interface.Interface):
... model = zope.schema.TextLine(title=u'Model')
... make = zope.schema.TextLine(title=u'Make')
... owner = zope.schema.Object(title=u'Owner', schema=IOwner)

Let's now implement the two interfaces and create instances, so that we can
create edit forms for it:

>>> class Owner(object):
... zope.interface.implements(IOwner)
... def __init__(self, name, license):
... self.name = name
... self.license = license

>>> class Car(object):
... zope.interface.implements(ICar)
... def __init__(self, model, make, owner):
... self.model = model
... self.make = make
... self.owner = owner

>>> me = Owner(u'Stephan Richter', u'MA-1231FW97')
>>> mycar = Car(u'Nissan', u'Sentra', me)

We define the owner sub-form as we would any other form. The only difference
is the template, which should not render a form-tag:

>>> import os
>>> from zope.app.pagetemplate import viewpagetemplatefile
>>> from z3c.form import form, field, tests

>>> templatePath = os.path.dirname(tests.__file__)

>>> class OwnerForm(form.EditForm):
... template = viewpagetemplatefile.ViewPageTemplateFile(
... 'simple_owneredit.pt', templatePath)
... fields = field.Fields(IOwner)
... prefix = 'owner'

Next we define the car form, which has the owner form as a sub-form. The car
form also needs a special template, since it needs to render the sub-form at
some point. For the simplicity of this example, I have duplicated a lot of
template code here, but you can use your favorite template techniques, such as
METAL macros, viewlets, or pagelets to make better reuse of some code.

>>> class CarForm(form.EditForm):
... fields = field.Fields(ICar).select('model', 'make')
... template = viewpagetemplatefile.ViewPageTemplateFile(
... 'simple_caredit.pt', templatePath)
... prefix = 'car'
... def update(self):
... self.owner = OwnerForm(self.context.owner, self.request)
... self.owner.update()
... super(CarForm, self).update()

Let's now instantiate the form and render it:

>>> from z3c.form.testing import TestRequest
>>> request = TestRequest()

>>> carForm = CarForm(mycar, request)
>>> carForm.update()
>>> print carForm.render()
<html>
<body>
<form action=".">
<div class="row">
<label for="car.widgets.model">Model</label>
<input type="text" id="car.widgets.model"
name="car.widgets.model" class="textWidget" value="Nissan" />
</div>
<div class="row">
<label for="car.widgets.make">Make</label>
<input type="text" id="car.widgets.make"
name="car.widgets.make" class="textWidget" value="Sentra" />
</div>
<fieldset>
<legend>Owner</legend>
<div class="row">
<label for="owner.widgets.name">Name</label>
<input type="text" id="owner.widgets.name"
name="owner.widgets.name" class="textWidget"
value="Stephan Richter" />
</div>
<div class="row">
<label for="owner.widgets.license">License</label>
<input type="text" id="owner.widgets.license"
name="owner.widgets.license" class="textWidget"
value="MA-1231FW97" />
</div>
<div class="action">
<input type="submit" id="owner.buttons.apply"
name="owner.buttons.apply" class="submitWidget"
value="Apply" />
</div>
</fieldset>
<div class="action">
<input type="submit" id="car.buttons.apply"
name="car.buttons.apply" class="submitWidget"
value="Apply" />
</div>
</form>
</body>
</html>

I can now submit the owner form, which should not submit any car changes I
might have made in the form:

>>> request = TestRequest(form={
... 'car.widgets.model': u'BMW',
... 'car.widgets.make': u'325',
... 'owner.widgets.name': u'Stephan Richter',
... 'owner.widgets.license': u'MA-97097A87',
... 'owner.buttons.apply': u'Apply'
... })

>>> carForm = CarForm(mycar, request)
>>> carForm.update()

>>> mycar.model
u'Nissan'
>>> mycar.make
u'Sentra'

>>> me.name
u'Stephan Richter'
>>> me.license
u'MA-97097A87'

Also, the form should say that the data of the owner has changed:

>>> print carForm.render()
<html>
<body>
<form action=".">
<div class="row">
<label for="car.widgets.model">Model</label>
<input type="text" id="car.widgets.model"
name="car.widgets.model" class="textWidget"
value="BMW" />
</div>
<div class="row">
<label for="car.widgets.make">Make</label>
<input type="text" id="car.widgets.make"
name="car.widgets.make" class="textWidget"
value="325" />
</div>
<fieldset>
<legend>Owner</legend>
<i>Data successfully updated.</i>
<div class="row">
<label for="owner.widgets.name">Name</label>
<input type="text" id="owner.widgets.name"
name="owner.widgets.name" class="textWidget"
value="Stephan Richter" />
</div>
<div class="row">
<label for="owner.widgets.license">License</label>
<input type="text" id="owner.widgets.license"
name="owner.widgets.license" class="textWidget"
value="MA-97097A87" />
</div>
<div class="action">
<input type="submit" id="owner.buttons.apply"
name="owner.buttons.apply" class="submitWidget"
value="Apply" />
</div>
</fieldset>
<div class="action">
<input type="submit" id="car.buttons.apply"
name="car.buttons.apply" class="submitWidget"
value="Apply" />
</div>
</form>
</body>
</html>

The same is true the other way around as well. Submitting the overall form
does not submit the owner form:

>>> request = TestRequest(form={
... 'car.widgets.model': u'BMW',
... 'car.widgets.make': u'325',
... 'car.buttons.apply': u'Apply',
... 'owner.widgets.name': u'Claudia Richter',
... 'owner.widgets.license': u'MA-123403S2',
... })

>>> carForm = CarForm(mycar, request)
>>> carForm.update()

>>> mycar.model
u'BMW'
>>> mycar.make
u'325'

>>> me.name
u'Stephan Richter'
>>> me.license
u'MA-97097A87'


Class II: The logical unit
--------------------------

In this class of sub-forms, a sub-form is often just a collection of widgets
without any actions. Instead, the sub-form must be able to react to the
actions of the parent form. A good example of those types of sub-forms is
actually the example I chose above.

So let's redevelop our example above in a way that the owner sub-form is just
a logical unit that shares the action with its parent form. Initially, the
example does not look very different, except that we use ``EditSubForm`` as a
base class:

>>> from z3c.form import subform

>>> class OwnerForm(subform.EditSubForm):
... template = viewpagetemplatefile.ViewPageTemplateFile(
... 'simple_subedit.pt', templatePath)
... fields = field.Fields(IOwner)
... prefix = 'owner'

The main form also is pretty much the same, except that a subform takes three
constructor arguments, the last one being the parent form:

>>> class CarForm(form.EditForm):
... fields = field.Fields(ICar).select('model', 'make')
... template = viewpagetemplatefile.ViewPageTemplateFile(
... 'simple_caredit.pt', templatePath)
... prefix = 'car'
...
... def update(self):
... super(CarForm, self).update()
... self.owner = OwnerForm(self.context.owner, self.request, self)
... self.owner.update()

Rendering the form works as before:

>>> request = TestRequest()
>>> carForm = CarForm(mycar, request)
>>> carForm.update()
>>> print carForm.render()
<html>
<body>
<form action=".">
<div class="row">
<label for="car.widgets.model">Model</label>
<input type="text" id="car.widgets.model" name="car.widgets.model"
class="textWidget" value="BMW" />
</div>
<div class="row">
<label for="car.widgets.make">Make</label>
<input type="text" id="car.widgets.make" name="car.widgets.make"
class="textWidget" value="325" />
</div>
<fieldset>
<legend>Owner</legend>
<div class="row">
<label for="owner.widgets.name">Name</label>
<input type="text" id="owner.widgets.name" name="owner.widgets.name"
class="textWidget" value="Stephan Richter" />
</div>
<div class="row">
<label for="owner.widgets.license">License</label>
<input type="text" id="owner.widgets.license"
name="owner.widgets.license" class="textWidget"
value="MA-97097A87" />
</div>
</fieldset>
<div class="action">
<input type="submit" id="car.buttons.apply"
name="car.buttons.apply" class="submitWidget"
value="Apply" />
</div>
</form>
</body>
</html>

The itnersting part of this setup is that the "Apply" button calls the action
handlers for both, the main and the sub-form:

>>> request = TestRequest(form={
... 'car.widgets.model': u'Ford',
... 'car.widgets.make': u'F150',
... 'car.buttons.apply': u'Apply',
... 'owner.widgets.name': u'Claudia Richter',
... 'owner.widgets.license': u'MA-991723FDG',
... })

>>> carForm = CarForm(mycar, request)
>>> carForm.update()

>>> mycar.model
u'Ford'
>>> mycar.make
u'F150'
>>> me.name
u'Claudia Richter'
>>> me.license
u'MA-991723FDG'

Let's now have a look at cases where an error happens. If an error occurs in
the parent form, the sub-form is still submitted:

>>> request = TestRequest(form={
... 'car.widgets.model': u'Volvo\n',
... 'car.widgets.make': u'450',
... 'car.buttons.apply': u'Apply',
... 'owner.widgets.name': u'Stephan Richter',
... 'owner.widgets.license': u'MA-991723FDG',
... })

>>> carForm = CarForm(mycar, request)
>>> carForm.update()

>>> mycar.model
u'Ford'
>>> mycar.make
u'F150'
>>> me.name
u'Stephan Richter'
>>> me.license
u'MA-991723FDG'

Let's look at the rendered form:

>>> print carForm.render()
<html>
<body>
<i>There were some errors.</i>
<ul>
<li>
Model: <div class="error">Constraint not satisfied</div>
</li>
</ul>
<form action=".">
<div class="row">
<b><div class="error">Constraint not satisfied</div>
</b><label for="car.widgets.model">Model</label>
<input type="text" id="car.widgets.model"
name="car.widgets.model" class="textWidget" value="Volvo " />
</div>
<div class="row">
<label for="car.widgets.make">Make</label>
<input type="text" id="car.widgets.make"
name="car.widgets.make" class="textWidget" value="450" />
</div>
<fieldset>
<legend>Owner</legend>
<i>Data successfully updated.</i>
<div class="row">
<label for="owner.widgets.name">Name</label>
<input type="text" id="owner.widgets.name"
name="owner.widgets.name" class="textWidget"
value="Stephan Richter" />
</div>
<div class="row">
<label for="owner.widgets.license">License</label>
<input type="text" id="owner.widgets.license"
name="owner.widgets.license" class="textWidget"
value="MA-991723FDG" />
</div>
</fieldset>
<div class="action">
<input type="submit" id="car.buttons.apply"
name="car.buttons.apply" class="submitWidget"
value="Apply" />
</div>
</form>
</body>
</html>

Now, we know, we know. This might not be the behavior that *you* want. But
remember how we started this document. We started with the recognition that
there are many classes and policies surrounding subforms. So while this
package provides some sensible default behavior, it is not intended to be
comprehensive.

Let's now create an error in the sub-form, ensuring that an error message
occurs:

>>> request = TestRequest(form={
... 'car.widgets.model': u'Volvo',
... 'car.widgets.make': u'450',
... 'car.buttons.apply': u'Apply',
... 'owner.widgets.name': u'Claudia\n Richter',
... 'owner.widgets.license': u'MA-991723F12',
... })

>>> carForm = CarForm(mycar, request)
>>> carForm.update()

>>> mycar.model
u'Volvo'
>>> mycar.make
u'450'
>>> me.name
u'Stephan Richter'
>>> me.license
u'MA-991723FDG'

>>> print carForm.render()
<html>
...
<fieldset>
<legend>Owner</legend>
<ul>
<li>
Name: <div class="error">Constraint not satisfied</div>
</li>
</ul>
...
</fieldset>
...
</html>

If the data did not change, it is also locally reported:

>>> request = TestRequest(form={
... 'car.widgets.model': u'Ford',
... 'car.widgets.make': u'F150',
... 'car.buttons.apply': u'Apply',
... 'owner.widgets.name': u'Stephan Richter',
... 'owner.widgets.license': u'MA-991723FDG',
... })

>>> carForm = CarForm(mycar, request)
>>> carForm.update()
>>> print carForm.render()
<html>
...
<fieldset>
<legend>Owner</legend>
<i>No changes were applied.</i>
...
</fieldset>
...
</html>

Final Note: With ``zope.formlib`` and ``zope.app.form`` people usually wrote
complex object widgets to handle objects within forms. We never considered
this a good way of programming, since one looses control over the layout too
easily.

==============
Field Managers
==============

One of the features in ``zope.formlib`` that works really well is the syntax
used to define the contents of the form. The formlib uses form fields, to
describe how the form should be put together. Since we liked this way of
working, this package offers this feature as well in a very similar way.

A field manager organizes all fields to be displayed within a form. Each field
is associated with additional meta-data. The simplest way to create a field
manager is to specify the schema from which to extract all fields.

Thus, the first step is to create a schema:

>>> import zope.interface
>>> import zope.schema

>>> class IPerson(zope.interface.Interface):
... id = zope.schema.Int(
... title=u'Id',
... readonly=True)
...
... name = zope.schema.TextLine(
... title=u'Name')
...
... country = zope.schema.Choice(
... title=u'Country',
... values=(u'Germany', u'Switzerland', u'USA'),
... required=False)

We can now create the field manager:

>>> from z3c.form import field
>>> manager = field.Fields(IPerson)

Like all managers in this package, the enumerable mapping API is provided:

>>> manager['id']
<Field 'id'>
>>> manager['unknown']
Traceback (most recent call last):
...
KeyError: 'unknown'

>>> manager.get('id')
<Field 'id'>
>>> manager.get('unknown', 'default')
'default'

>>> 'id' in manager
True
>>> 'unknown' in manager
False

>>> manager.keys()
['id', 'name', 'country']

>>> [key for key in manager]
['id', 'name', 'country']

>>> manager.values()
[<Field 'id'>, <Field 'name'>, <Field 'country'>]

>>> manager.items()
[('id', <Field 'id'>),
('name', <Field 'name'>),
('country', <Field 'country'>)]

>>> len(manager)
3

You can also select the fields that you would like to have:

>>> manager = manager.select('name', 'country')
>>> manager.keys()
['name', 'country']

Changing the order is simply a matter of changing the selction order:

>>> manager = manager.select('country', 'name')
>>> manager.keys()
['country', 'name']

Sometimes it is easier to simply omit a set of fields instead of selecting all
the ones you want:

>>> manager = field.Fields(IPerson)
>>> manager = manager.omit('id')
>>> manager.keys()
['name', 'country']

You can also add two field managers together:

>>> manager2 = field.Fields(IPerson).select('id')
>>> (manager + manager2).keys()
['name', 'country', 'id']

But adding anything else to a field manager is not well defined:

>>> manager + 1
Traceback (most recent call last):
...
TypeError: unsupported operand type(s) for +: 'Fields' and 'int'

You can also not make any additions that would cause a name conflict:

>>> manager + manager
Traceback (most recent call last):
...
ValueError: ('Duplicate name', 'name')

When creating a new form derived from another, you often want to keep existing
fields and add new ones. In order no to change the super-form class, you need
to copy the field manager:

>>> manager.keys()
['name', 'country']
>>> manager.copy().keys()
['name', 'country']


More on the Constructor
-----------------------

But the constructor does not only accept schemas to be passed in. One can also
just pass in schema fields:

>>> field.Fields(IPerson['name']).keys()
['name']

However, the schema field has to have a name:

>>> email = zope.schema.TextLine(title=u'E-Mail')
>>> field.Fields(email)
Traceback (most recent call last):
...
ValueError: Field has no name

Adding a name helps:

>>> email.__name__ = 'email'
>>> field.Fields(email).keys()
['email']

Or you can just pass in other field managers, which is the feature the add
mechanism uses:

>>> field.Fields(manager).keys()
['name', 'country']

Last, but not least, the constructor also accepts form fields, which is used
by ``select()`` and ``omit()``:

>>> field.Fields(manager['name'], manager2['id']).keys()
['name', 'id']

If the constructor does not recognize any of the types above, it raises a
``TypeError`` exception:

>>> field.Fields(object())
Traceback (most recent call last):
...
TypeError: ('Unrecognized argument type', <object object at ...>)

Additionally, you can specify several keyword arguments in the field manager
constructor that are used to setup the fields:

* ``omitReadOnly``

When set to ``True`` all read-only fields are omitted.

>>> field.Fields(IPerson, omitReadOnly=True).keys()
['name', 'country']

* ``keepReadOnly``

Sometimes you want to keep a particular read-only field around, even though
in general you want to omit them. In this case you can specify the fields to
keep:

>>> field.Fields(
... IPerson, omitReadOnly=True, keepReadOnly=('id',)).keys()
['id', 'name', 'country']

* ``prefix``

Sets the prefix of the fields. This argument is passed on to each field.

>>> manager = field.Fields(IPerson, prefix='myform.')
>>> manager['myform.name']
<Field 'myform.name'>


* ``interface``

Usually the interface is inferred from the field itself. The interface is
used to determine whether an adapter must be looked up for a given
context.

But sometimes fields are generated in isolation to an interface or the
interface of the field is not the one you want. In this case you can specify
the interface:

>>> class IMyPerson(IPerson):
... pass

>>> manager = field.Fields(email, interface=IMyPerson)
>>> manager['email'].interface
<InterfaceClass __builtin__.IMyPerson>

* ``mode``

The mode in which the widget will be rendered. By default there are two
available, "input" and "display". When mode is not specified, "input" is
chosen.

>>> from z3c.form import interfaces
>>> manager = field.Fields(IPerson, mode=interfaces.DISPLAY_MODE)
>>> manager['country'].mode
'display'


Fields Widget Manager
---------------------

When a form (or any other widget-using view) is updated, one of the tasks is
to create the widgets. Traditionally, generating the widgets involved looking
at the form fields (or similar) of a form and generating the widgets using the
information of those specifications. This solution is good for the common
(about 85%) use cases, since it makes writing new forms very simple and allows
a lot of control at a class-definition level.

It has, however, its limitations. It does not, for example, allow for
customization without rewriting a form. This can range from omitting fields on
a particular form to generically adding a new widget to the form, such as an
"object name" button on add forms. This package solves this issue by providing
a widget manager, which is responsible providing the widgets for a particular
view.

The default widget manager for forms is able to look at a form's field
definitions and create widgets for them. Thus, let's create a schema first:

>>> import zope.interface
>>> import zope.schema

>>> class LastNameTooShort(zope.schema.interfaces.ValidationError):
... """The last name is too short."""

>>> class IPerson(zope.interface.Interface):
... lastName = zope.schema.TextLine(
... title=u'Last Name',
... description=u"The person's last name.",
... default=u'',
... required=True)
...
... firstName = zope.schema.TextLine(
... title=u'First Name',
... description=u"The person's first name.",
... default=u'-- unknown --',
... required=False)
...
... @zope.interface.invariant
... def twiceAsLong(person):
... if len(person.lastName) >= 2 * len(person.firstName):
... raise LastNameTooShort()

Next we need a form that specifies the fields to be added:

>>> from z3c.form import field

>>> class AddPerson(object):
... prefix = 'form.'
... fields = field.Fields(IPerson)
>>> addPerson = AddPerson()

For more details on how to define fields within a form, see ``field.txt``. We
can now create the fields widget manager. It's discriminators are the form for
which the widgets are created, the request, and the context that is being
manipulated. Since we are developing an add-form the context is ``None``.

>>> from z3c.form.testing import TestRequest
>>> request = TestRequest()
>>> context = object()

>>> manager = field.FieldWidgets(addPerson, request, context)
>>> manager.ignoreContext = True

The main resposibility of the manager is to provide the ``IEnumerableMapping``
interface and an ``update()`` method. Initially the mapping, going from widget
id to widget value, is empty:

>>> from zope.interface.common.mapping import IEnumerableMapping
>>> IEnumerableMapping.providedBy(manager)
True

>>> manager.keys()
[]

Only by "updating" the manager, the widgets will become available. But before
we can use the update method we have to register the ``IFieldWidget`` adapter
for the ``ITextLine`` field:

>>> from z3c.form import interfaces, widget

>>> @zope.component.adapter(zope.schema.TextLine, TestRequest)
... @zope.interface.implementer(interfaces.IFieldWidget)
... def TextFieldWidget(field, request):
... return widget.FieldWidget(field, widget.Widget(request))

>>> zope.component.provideAdapter(TextFieldWidget)

>>> from z3c.form import converter
>>> zope.component.provideAdapter(converter.FieldDataConverter)
>>> zope.component.provideAdapter(converter.FieldWidgetDataConverter)

>>> manager.update()

Other than usual mappings in Python, the widget manager's widgets are always
in a particular order:

>>> manager.keys()
['lastName', 'firstName']

Let's make sure that all enumerable mapping functions work correctly:

>>> manager['lastName']
<Widget 'form.widgets.lastName'>

>>> manager['unknown']
Traceback (most recent call last):
...
KeyError: 'unknown'

>>> manager.get('lastName')
<Widget 'form.widgets.lastName'>

>>> manager.get('unknown', 'default')
'default'

>>> 'lastName' in manager
True
>>> 'unknown' in manager
False

>>> [key for key in manager]
['lastName', 'firstName']

>>> manager.values()
[<Widget 'form.widgets.lastName'>,
<Widget 'form.widgets.firstName'>]

>>> manager.items()
[('lastName', <Widget 'form.widgets.lastName'>),
('firstName', <Widget 'form.widgets.firstName'>)]

>>> len(manager)
2

When a widget is added to the widget manager, it is located:

>>> lname = manager['lastName']

>>> lname.__name__
'lastName'
>>> lname.__parent__
<z3c.form.field.FieldWidgets object at ...>

All widgets created by this widget manager are context aware:

>>> interfaces.IContextAware.providedBy(lname)
True
>>> lname.context is context
True

All widgets will also assume the mode of the manager:

>>> manager['lastName'].mode
'input'

>>> manager.mode = interfaces.DISPLAY_MODE
>>> manager.update()

>>> manager['lastName'].mode
'display'

Besides managing widgets, the widget manager also controls the process of
extracting and validating extracted data. Let's start with the validation
first, which only validates the data as a whole, assuming each individual
value being already validated.

Before we can use the method, we have to register a "manager validator":

>>> from z3c.form import validator
>>> zope.component.provideAdapter(validator.InvariantsValidator)

>>> manager.validate(
... {'firstName': u'Stephan', 'lastName': u'Richter'})
()

The result of this method is a tuple of errors that occured during the
validation. An empty tuple means the validation succeeded. Let's now make the
validation fail:

>>> errors = manager.validate(
... {'firstName': u'Stephan', 'lastName': u'Richter-Richter'})

>>> [error.doc() for error in errors]
['The last name is too short.']

A special case occurs when the schema fields are not associated with an
interface:

>>> name = zope.schema.TextLine(__name__='name')

>>> class PersonForm(object):
... prefix = 'form.'
... fields = field.Fields(name)
>>> personForm = PersonForm()

>>> manager = field.FieldWidgets(personForm, request, context)

In this case, the widget manager's ``validate()`` method should simply ignore
the field and not try to look up any invariants:

>>> manager.validate({'name': u'Stephan'})
()

Let's now have a look at the widget manager's ``extract()``, which returns a
data dictionary and the collection of errors. Before we can validate, we have
to register a validator for the widget:

>>> zope.component.provideAdapter(validator.SimpleFieldValidator)

When all goes well, the data dictionary is complete and the error collection
empty:

>>> request = TestRequest(form={
... 'form.widgets.firstName': u'Stephan',
... 'form.widgets.lastName': u'Richter'})
>>> manager = field.FieldWidgets(addPerson, request, context)
>>> manager.ignoreContext = True
>>> manager.update()
>>> manager.extract()
({'lastName': u'Richter', 'firstName': u'Stephan'}, ())

Since all errors are immediately converted to error view snippets, we have to
provide the adapter from a validation error to an error view snippet first:

>>> from z3c.form import error
>>> zope.component.provideAdapter(error.ErrorViewSnippet)

Let's now cause a widget-level error by not submitting the required last
name:

>>> request = TestRequest(form={
... 'form.widgets.firstName': u'Stephan'})
>>> manager = field.FieldWidgets(addPerson, request, context)
>>> manager.ignoreContext = True
>>> manager.update()
>>> manager.extract()
({'firstName': u'Stephan'}, (<ErrorViewSnippet for RequiredMissing>,))

Finally, let's ensure that invariant failures are also caught:

>>> request = TestRequest(form={
... 'form.widgets.firstName': u'Stephan',
... 'form.widgets.lastName': u'Richter-Richter'})
>>> manager = field.FieldWidgets(addPerson, request, context)
>>> manager.ignoreContext = True
>>> manager.update()
>>> data, errors = manager.extract()
>>> errors[0].doc()
'The last name is too short.'

And that's all.


Fields -- Custom Widget Factories
---------------------------------

It is possible to declare custom widgets for fields within the field's
declaration.

Let's have a look at the default form first. Initially, the standard
registered widgets are used:

>>> manager = field.FieldWidgets(addPerson, request, context)
>>> manager.update()

>>> manager['firstName']
<Widget 'form.widgets.firstName'>

Now we would like to have our own custom input widget:

>>> class CustomInputWidget(widget.Widget):
... pass

>>> def CustomInputWidgetFactory(field, request):
... return widget.FieldWidget(field, CustomInputWidget(request))

It can be simply assigned as follows:

>>> addPerson.fields['firstName'].widgetFactory = CustomInputWidgetFactory

Now this widget should be used instead of the registered default one:

>>> manager = field.FieldWidgets(addPerson, request, context)
>>> manager.update()
>>> manager['firstName']
<CustomInputWidget 'form.widgets.firstName'>

In the background the widget factory assignment really just registered the
default factory for in the ``WidgetFactories`` object, which manages the
custom widgets for all modes. Now all modes show this input widget:

>>> manager = field.FieldWidgets(addPerson, request, context)
>>> manager.mode = interfaces.DISPLAY_MODE
>>> manager.update()
>>> manager['firstName']
<CustomInputWidget 'form.widgets.firstName'>

However, we can also register a specific widget for the display mode:

>>> class CustomDisplayWidget(widget.Widget):
... pass

>>> def CustomDisplayWidgetFactory(field, request):
... return widget.FieldWidget(field, CustomDisplayWidget(request))

>>> addPerson.fields['firstName']\
... .widgetFactory[interfaces.DISPLAY_MODE] = CustomDisplayWidgetFactory

Now the display mode should produce the custom display widget, ...

>>> manager = field.FieldWidgets(addPerson, request, context)
>>> manager.mode = interfaces.DISPLAY_MODE
>>> manager.update()
>>> manager['firstName']
<CustomDisplayWidget 'form.widgets.firstName'>

... while the input mode still shows the default custom input widget:

>>> manager = field.FieldWidgets(addPerson, request, context)
>>> manager.mode = interfaces.INPUT_MODE
>>> manager.update()
>>> manager['firstName']
<CustomInputWidget 'form.widgets.firstName'>

The widgets factories component,

>>> factories = addPerson.fields['firstName'].widgetFactory
>>> factories
{'display': <function CustomDisplayWidgetFactory at ...>}

is pretty much a standard dictionary that also manages a default value:

>>> factories.default
<function CustomInputWidgetFactory at ...>

When getting a value for a key, if the key is not found, the default is
returned:

>>> factories.keys()
['display']

>>> factories[interfaces.DISPLAY_MODE]
<function CustomDisplayWidgetFactory at ...>
>>> factories[interfaces.INPUT_MODE]
<function CustomInputWidgetFactory at ...>

>>> factories.get(interfaces.DISPLAY_MODE)
<function CustomDisplayWidgetFactory at ...>
>>> factories.get(interfaces.INPUT_MODE)
<function CustomInputWidgetFactory at ...>

If no default is specified,

>>> factories.default = None

then the dictionary behaves as usual:

>>> factories[interfaces.DISPLAY_MODE]
<function CustomDisplayWidgetFactory at ...>
>>> factories[interfaces.INPUT_MODE]
Traceback (most recent call last):
...
KeyError: 'input'

>>> factories.get(interfaces.DISPLAY_MODE)
<function CustomDisplayWidgetFactory at ...>
>>> factories.get(interfaces.INPUT_MODE)


=======
Buttons
=======

Buttons are a method to declare actions for a form. Like fields describe
widgets within a form, buttons describe actions. The symmetry goes even
further; like fields, buttons are schema fields within schema. When the form
is instantiated and updated, the buttons are converted to actions.

>>> from z3c.form import button


Schema Defined Buttons
----------------------

Let's now create a schema that describes the buttons of a form. Having button
schemas allows one to more easily reuse button declarations and to group them
logically. ``Button`` objects are just a simple extension to ``Field``
objects, so they behave identical within a schema:

>>> import zope.interface
>>> class IButtons(zope.interface.Interface):
... apply = button.Button(title=u'Apply')
... cancel = button.Button(title=u'Cancel')

In reality, only the title and name is relevant. Let's now create a form that
provides those buttons.

>>> from z3c.form import interfaces
>>> class Form(object):
... zope.interface.implements(
... interfaces.IButtonForm, interfaces.IHandlerForm)
... buttons = button.Buttons(IButtons)
... prefix = 'form'
...
... @button.handler(IButtons['apply'])
... def apply(self, action):
... print 'successfully applied'
...
... @button.handler(IButtons['cancel'])
... def cancel(self, action):
... self.request.response.redirect('index.html')

Let' now create an action manager for the button manager in the form. To do
that we first need a request and a form instance:

>>> from z3c.form.testing import TestRequest
>>> request = TestRequest()
>>> form = Form()

Action managers are instantiated using the form, request, and
context/content. A special button-action-manager implementation is avaialble
in the ``button`` package:

>>> actions = button.ButtonActions(form, request, None)
>>> actions.update()

Once the action manager is updated, the buttons should be available as
actions:

>>> actions.keys()
['apply', 'cancel']
>>> actions['apply']
<ButtonAction 'form.buttons.apply' u'Apply'>

Button actions are locations:

>>> apply = actions['apply']
>>> apply.__name__
'apply'
>>> apply.__parent__
<ButtonActions None>

A button action is also a submit widget. The attributes translate as follows:

>>> interfaces.ISubmitWidget.providedBy(apply)
True

>>> apply.value == apply.title
True
>>> apply.id == apply.name
True

Next we want to display our button actions. To be able to do this, we have to
register a template for the submit widget:

>>> from z3c.form import testing, widget
>>> templatePath = testing.getPath('submit_input.pt')
>>> factory = widget.WidgetTemplateFactory(templatePath, 'text/html')

>>> from zope.pagetemplate.interfaces import IPageTemplate
>>> zope.component.provideAdapter(factory,
... (zope.interface.Interface, TestRequest, None, None,
... interfaces.ISubmitWidget),
... IPageTemplate, name='input')

A widget template has many discriminators: context, request, view, field, and
widget. We can now render each action:

>>> print actions['apply'].render()
<input type="submit" id="form.buttons.apply"
name="form.buttons.apply" class="submitWidget"
value="Apply" />

So displaying is nice, but how do button handlers get executed? The action
manager provides attributes and method to check whether actions were
executed. Initially there are no executed actions:

>>> list(actions.executedActions)
[]

So in this case executing the actions does not do anything:

>>> actions.execute()

But if the request contains the information that the button was pressed, the
execution works:

>>> request = TestRequest(form={'form.buttons.apply': 'Apply'})

>>> actions = button.ButtonActions(form, request, None)
>>> actions.update()
>>> actions.execute()

Aehm, something should have happened. But in order for the system to look at
the handlers declared in the form, a special action handler has to be
registered with the system:

>>> zope.component.provideAdapter(button.ButtonActionHandler)

And voila, the execution works:

>>> actions.execute()
successfully applied

Finally, if there is no handler for a button, then the button click is
silently ignored:

>>> form.handlers = button.Handlers()
>>> actions.execute()

While this might seem awkward at first, this is an intended feature. Sometimes
there are several sub-forms that listen to a particular button and one form or
another might simply not care about the button at all and not provide a
handler.


In-Form Button Declarations
---------------------------

Some readers might find it cumbersome to declare a full schema just to create
some buttons. A faster method is to write simple arguments to the button
manager:

>>> class Form(object):
... zope.interface.implements(
... interfaces.IButtonForm, interfaces.IHandlerForm)
... buttons = button.Buttons(
... button.Button('apply', title=u'Apply'))
... prefix = 'form.'
...
... @button.handler(buttons['apply'])
... def apply(self, action):
... print 'successfully applied'

The first argument of the ``Button`` class constructor is the name of the
button. Optionally, this can also be one of the follwoing keyword arguments:

>>> button.Button(name='apply').__name__
'apply'
>>> button.Button(__name__='apply').__name__
'apply'

If no name is specified, the name will be assigned to the button. When it can
it uses the title, otherwise a hexadecimal string is created:

>>> button.Button(title=u'Apply').__name__
'apply'
>>> button.Button(title=u'Apply and more').__name__
'4170706c7920616e64206d6f7265'


This declaration behaves identical to the one before:

>>> form = Form()
>>> request = TestRequest()

>>> actions = button.ButtonActions(form, request, None)
>>> actions.update()
>>> actions.execute()

When sending in the right information, the actions are executed:

>>> request = TestRequest(form={'form.buttons.apply': 'Apply'})
>>> actions = button.ButtonActions(form, request, None)
>>> actions.update()
>>> actions.execute()
successfully applied

An even simpler method -- resembling closest the API provided by formlib -- is
to create the button and handler at the same time:

>>> class Form(object):
... zope.interface.implements(
... interfaces.IButtonForm, interfaces.IHandlerForm)
... prefix = 'form.'
...
... @button.buttonAndHandler(u'Apply')
... def apply(self, action):
... print 'successfully applied'

In this case the ``buttonAndHandler`` decorator creates a button and a handler
for it. By default the name is computed from the title of the button, which is
required. All (keyword) arguments are forwarded to the button
constructor. Let's now render the form:

>>> request = TestRequest(form={'form.buttons.apply': 'Apply'})
>>> actions = button.ButtonActions(form, request, None)
>>> actions.update()
>>> actions.execute()
successfully applied

If the title is a more complex string, then the name of the button becomes a
hex-encoded string:

>>> class Form(object):
...
... @button.buttonAndHandler(u'Apply and Next')
... def apply(self, action):
... print 'successfully applied'

>>> Form.buttons.keys()
['4170706c7920616e64204e657874']

Of course, you can use the ``__name__`` argument to specify a nem
yourself. The decorator, however, also allows the keyword ``name``:

>>> class Form(object):
...
... @button.buttonAndHandler(u'Apply and Next', name='applyNext')
... def apply(self, action):
... print 'successfully applied'

>>> Form.buttons.keys()
['applyNext']

This helper function also supports a keyword argument ``provides``, which
allows the developer to specify a sequence of interfaces that the generated
button should directly provide. Those provided interfaces can be used for a
multitude of things, including handler discrimination and UI layout:

>>> class IMyButton(zope.interface.Interface):
... pass

>>> class Form(object):
...
... @button.buttonAndHandler(u'Apply', provides=(IMyButton,))
... def apply(self, action):
... print 'successfully applied'

>>> IMyButton.providedBy(Form.buttons['apply'])
True


Button Conditions
-----------------

Sometimes it is desirable to only show a button when a certain condition is
fulfilled. The ``Button`` field supports conditions via a simple argument. The
``condition`` argument must be a callable takening the form as argument and
returns a truth-value. If the condition is not fulfilled, the button will not
be converted to an action:

>>> class Form(object):
... prefix='form'
... showApply = True
...
... @button.buttonAndHandler(
... u'Apply', condition=lambda form: form.showApply)
... def apply(self, action):
... print 'successfully applied'

In this case a form variable specifies the availability. Initially the button
is available as action:

>>> myform = Form()
>>> actions = button.ButtonActions(myform, TestRequest(), None)
>>> actions.update()
>>> actions.keys()
['apply']

If we set the show-apply attribute to false, the action will not be available.

>>> myform.showApply = False
>>> actions = button.ButtonActions(myform, TestRequest(), None)
>>> actions.update()
>>> actions.keys()
[]

This feature is very helpful in multi-forms and wizards.


Customizing the Title
---------------------

As for widgets, it is often desirable to change attributes of the button
actions without altering any original code. Again we will be using attribute
value adapters to complete the task. Originally, our title is as follows:

>>> myform = Form()
>>> actions = button.ButtonActions(myform, TestRequest(), None)
>>> actions.update()
>>> actions['apply'].title
u'Apply'

Let's now create a custom label for the action:

>>> ApplyLabel = button.StaticButtonActionAttribute(
... u'Apply now', button=myform.buttons['apply'])
>>> zope.component.provideAdapter(ApplyLabel, name='title')

Once the button action manager is updated, the new tite is chosen:

>>> actions.update()
>>> actions['apply'].title
u'Apply now'


The Button Manager
------------------

The button manager contains several additional API methods that make the
management of buttons easy.

First, you are able to add button managers:

>>> bm1 = button.Buttons(IButtons)
>>> bm2 = button.Buttons(button.Button('help', title=u'Help'))

>>> bm1 + bm2
<z3c.form.button.Buttons object at ...>
>>> list(bm1 + bm2)
['apply', 'cancel', 'help']

The result of the addition is another button manager. Also note that the order
of the buttons is preserved throughout the addition. Adding anything else is
not well-defined:

>>> bm1 + 1
Traceback (most recent call last):
...
TypeError: unsupported operand type(s) for +: 'Buttons' and 'int'

Second, you can select the buttons in a particular order:

>>> bm = bm1 + bm2
>>> list(bm)
['apply', 'cancel', 'help']

>>> list(bm.select('help', 'apply', 'cancel'))
['help', 'apply', 'cancel']

The ``select()`` method can also be used to eliminate another button:

>>> list(bm.select('help', 'apply'))
['help', 'apply']

Of course, in the example above we elimintated one and reoganized the buttons.

Third, you can omit one or more buttons:

>>> list(bm.omit('cancel'))
['apply', 'help']

Finally, while the constructor is very flexible, you cannot just pass in
anything:

>>> button.Buttons(1, 2)
Traceback (most recent call last):
...
TypeError: ('Unrecognized argument type', 1)

When creating a new form derived from another, you often want to keep existing
buttons and add new ones. In order no to change the super-form class, you need
to copy the button manager:

>>> bm.keys()
['apply', 'cancel', 'help']
>>> bm.copy().keys()
['apply', 'cancel', 'help']


The Handlers Object
-------------------

All handlers of a form are collected in the ``handlers`` attribute, which is a
``Handlers`` instance:

>>> isinstance(form.handlers, button.Handlers)
True
>>> form.handlers
<Handlers [<Handler for <Button 'apply' u'Apply'>>]>

Internally the object uses an adapter registry to manage the handlers for
buttons. If a handler is registered for a button, it simply behaves as an
instance-adapter.

The object itself is pretty simple. You can get a handler as follows:

>>> apply = form.buttons['apply']
>>> form.handlers.getHandler(apply)
<Handler for <Button 'apply' u'Apply'>>

But you can also register handlers for groups of buttons, either by interface
or class:

>>> class SpecialButton(button.Button):
... pass

>>> def handleSpecialButton(form, action):
... return 'Special button action'

>>> form.handlers.addHandler(
... SpecialButton, button.Handler(SpecialButton, handleSpecialButton))

>>> form.handlers
<Handlers
[<Handler for <Button 'apply' u'Apply'>>,
<Handler for <class 'SpecialButton'>>]>

Now all special buttons should use that handler:

>>> button1 = SpecialButton(title=u'Button 1')
>>> button2 = SpecialButton(title=u'Button 2')

>>> form.handlers.getHandler(button1)(form, None)
'Special button action'
>>> form.handlers.getHandler(button2)(form, None)
'Special button action'

However, registering a more specific handler for button 1 will override the
general handler:

>>> def handleButton1(form, action):
... return 'Button 1 action'

>>> form.handlers.addHandler(
... button1, button.Handler(button1, handleButton1))

>>> form.handlers.getHandler(button1)(form, None)
'Button 1 action'
>>> form.handlers.getHandler(button2)(form, None)
'Special button action'

You can also add handlers objects:

>>> handlers2 = button.Handlers()

>>> button3 = SpecialButton(title=u'Button 3')
>>> handlers2.addHandler(
... button3, button.Handler(button3, None))

>>> form.handlers + handlers2
<Handlers
[<Handler for <Button 'apply' u'Apply'>>,
<Handler for <class 'SpecialButton'>>,
<Handler for <SpecialButton '427574746f6e2031' u'Button 1'>>,
<Handler for <SpecialButton '427574746f6e2033' u'Button 3'>>]>

However, adding other components is not supported:

>>> form.handlers + 1
Traceback (most recent call last):
...
NotImplementedError

The handlers also provide a method to copy the handlers to a new instance:

>>> copy = form.handlers.copy()
>>> isinstance(copy, button.Handlers)
True
>>> copy is form.handlers
False

This is commonly needed when one wants to extend the handlers of a super-form.

==========
Directives
==========

Widget template directive
-------------------------

Show how we can use the widget template directive. Register the meta
configuration for the directive.

>>> import sys
>>> from zope.configuration import xmlconfig
>>> import z3c.form
>>> context = xmlconfig.file('meta.zcml', z3c.form)

We need a custom widget template

>>> import os, tempfile
>>> temp_dir = tempfile.mkdtemp()
>>> file = os.path.join(temp_dir, 'widget.pt')
>>> open(file, 'w').write('''
... <input type="text" id="" name="" value="" size=""
... tal:attributes="id view/id;
... name view/name;
... size view/size;
... value view/value;" />
... ''')

and a interface

>>> import zope.interface
>>> from z3c.form import interfaces
>>> class IMyWidget(interfaces.IWidget):
... """My widget interface."""

and a widget class:

>>> from z3c.form.testing import TestRequest
>>> from z3c.form.browser import text
>>> class MyWidget(text.TextWidget):
... zope.interface.implements(IMyWidget)
>>> request = TestRequest()
>>> myWidget = MyWidget(request)

Make them available under the fake package ``custom``:

>>> sys.modules['custom'] = type(
... 'Module', (),
... {'IMyWidget': IMyWidget})()

and register them as a widget template within the ``z3c:widgetTemplate``
directive:

>>> context = xmlconfig.string("""
... <configure
... xmlns:z3c="http://namespaces.zope.org/z3c">
... <z3c:widgetTemplate
... template="%s"
... widget="custom.IMyWidget"
... />
... </configure>
... """ % file, context=context)

Let's get the template

>>> import zope.component
>>> from z3c.template.interfaces import IPageTemplate
>>> template = zope.component.queryMultiAdapter((None, request, None, None,
... myWidget), interface=IPageTemplate, name='input')

and check them:

>>> template
<zope.app.pagetemplate.viewpagetemplatefile.ViewPageTemplateFile ...>

Let's use the template within the widget.

>>> print template(myWidget)
<input type="text" id="" name="" value="" />

We normly render the widget which returns the registered template.

>>> print myWidget.render()
<input type="text" id="" name="" value="" />

If the template does not exist, then the widget directive should fail
immediately:

>>> unknownFile = os.path.join(temp_dir, 'unknown.pt')
>>> context = xmlconfig.string("""
... <configure
... xmlns:z3c="http://namespaces.zope.org/z3c">
... <z3c:widgetTemplate
... template="%s"
... widget="custom.IMyWidget"
... />
... </configure>
... """ % unknownFile, context=context)
Traceback (most recent call last):
...
ZopeXMLConfigurationError: File "<string>", line 4.2-7.8
ConfigurationError: ('No such file', '...unknown.pt')


Cleanup
-------

Now we need to clean up the custom module.

>>> del sys.modules['custom']

==========
Validators
==========

Validators are components that validate submitted data. This is certainly not
a new concept, but in the previous form frameworks validation was hidden in
many places:

* Field/Widget Validation

The schema field consists of a ``validate()`` method. Validation is
automatically invoked when converting a unicode string to a field value
using ``fromUnicode()``. This makes it very hard to customize the field
validation. No hooks were provided to exert dditional restriction at the
presentation level.

* Schema/Form Validation

This type of validation was not supported at all initially. ``zope.formlib``
fixed this problem by validating against schema invariants. While this was a
first good step, it still made it hard to customize validators, since it
required touching the base implementations of the forms.

* Action Validation

``zope.formlib`` supports the notion of action validatos. Actions have a
success and failure handler. If the validation succeeds, the success handler
is called, otherwise the failure handler is chosen. We believe that this
design was ill-conceived, especially the default, which required the data to
completely validate in order for the action to successful. There are many
actions that do not even care about the data in the form, such as "Help",
"Cancel" and "Reset" buttons. Thus validation should be part of the data
retrieval process and not the action.

For me, the primary goals of the validator framework are as follows:

* Assert additional restrictions on the data at the presentation
level.

There are several use cases for this. Sometimes clients desire additional
restrictions on data for their particular version of the software. It is not
always desireable to adjust the model for this client, since the framework
knows how to handle the less restrictive case anyways. In another case,
additional restrictions might be applied to a particular form due to limited
restrictions.

* Make validation pluggable.

Like most other components of this package, it should be possible to control
the validation adapters at a fine grained level.

* Widgets: context, request, view, field[1], widget

* Widget Managers: context, request, view, schema[2], manager

[1].. This is optional, since widgets must not necessarily have fields.
[2].. This is optional, since widget managers must not necessarily have
manage field widgets and thus know about schemas.

* Provide good defaults that behave sensibly.

God defaults are, like in anywhere in this pacakge, very important. We have
chosen to implement the ``zope.formlib`` behavior as the default, since it
worked very well -- with exception of action validation, of course.

For this package, we have decided to support validators at the widget and
widget manager level. By default the framework only supports field widgets,
since the validation of field-absent widgets is generally not
well-defined. Thus, we first need to create a schema.

>>> import zope.interface
>>> import zope.schema
>>> class IPerson(zope.interface.Interface):
... login = zope.schema.TextLine(
... title=u'Login',
... min_length=1,
... max_length=10)
...
... email = zope.schema.TextLine(
... title=u'E-mail')
...
... @zope.interface.invariant
... def isLoginPartOfEmail(person):
... if not person.email.startswith(person.login):
... raise zope.interface.Invalid("The login not part of email.")


Widget Validators
-----------------

Widget validators only validate the data of one particular widget. The
validated value is always assumed to be an internal value and not a widget
value.

By default, the system uses the simple field validator, which simply uses the
``validate()`` method of the field. For instantiation, all validators have the
following signature for its discriminators: context, request, view, field, and
widget

>>> from z3c.form import validator
>>> simple = validator.SimpleFieldValidator(
... None, None, None, IPerson['login'], None)

A validator has a single method ``validate()``. When the validation is
successful, ``None`` is returned:

>>> simple.validate(u'srichter')

A validation error is raised, when the validation fails:

>>> simple.validate(u'StephanCaveman3')
Traceback (most recent call last):
...
TooLong: (u'StephanCaveman3', 10)

Let's now create a validator that also requires at least 1 numerical character
in the login name:

>>> import re

>>> class LoginValidator(validator.SimpleFieldValidator):
...
... def validate(self, value):
... super(LoginValidator, self).validate(value)
... if re.search('[0-9]', value) is None:
... raise zope.interface.Invalid('No numerical character found.')

Let's now try our new validator:

>>> login = LoginValidator(None, None, None, IPerson['login'], None)

>>> login.validate(u'srichter1')

>>> login.validate(u'srichter')
Traceback (most recent call last):
...
Invalid: No numerical character found.

We can now register the validator with the component architecture, ...

>>> import zope.component
>>> zope.component.provideAdapter(LoginValidator)

and look up the adapter using the usual way:

>>> from z3c.form import interfaces

>>> zope.component.queryMultiAdapter(
... (None, None, None, IPerson['login'], None),
... interfaces.IValidator)
<LoginValidator for IPerson['login']>

Unfortunately, the adapter is now registered for all fields, so that the
E-mail field also has this restriction (which is okay in this case, but not
generally):

>>> zope.component.queryMultiAdapter(
... (None, None, None, IPerson['email'], None),
... interfaces.IValidator)
<LoginValidator for IPerson['email']>

The validator module provides a helper function to set the discriminators for
a validator, which can include instances:

>>> validator.WidgetValidatorDiscriminators(
... LoginValidator, field=IPerson['login'])

Let's now clean up the component architecture and register the login validator
again:

>>> from zope.testing import cleanup
>>> cleanup.cleanUp()

>>> zope.component.provideAdapter(LoginValidator)

>>> zope.component.queryMultiAdapter(
... (None, None, None, IPerson['login'], None),
... interfaces.IValidator)
<LoginValidator for IPerson['login']>

>>> zope.component.queryMultiAdapter(
... (None, None, None, IPerson['email'], None),
... interfaces.IValidator)


Widget Manager Validators
-------------------------

The widget manager validator, while similar in spirit, works somewhat
different. The discriminators of the widget manager validator are: context,
request, view, schema, and manager.

A simple default implementation is provided that checks the invariants of the
schemas:

>>> invariants = validator.InvariantsValidator(
... None, None, None, IPerson, None)

Widget manager validators have the option to validate a data dictionary,

>>> invariants.validate(
... {'login': u'srichter', 'email': u'[e-mail]'})
()

or an object implementing the schema:

>>> class Person(object):
... login = u'srichter'
... email = u'[e-mail]'
>>> stephan = Person()

>>> invariants.validateObject(stephan)
()

Since multiple errors can occur during the validation process, all errors are
collected in a tuple, which is returned. If the tuple is empty, the validation
was successful. Let's now generate a failure:

>>> errors = invariants.validate(
... {'login': u'srichter', 'email': u'[e-mail]'})

>>> errors
(<zope.interface.exceptions.Invalid instance at ...>,)

>>> str(errors[0])
'The login not part of email.'

Let's now have a look at writing a custom validator. In this case, we want to
ensure that the E-mail address is at most twice as long as the login:

>>> class CustomValidator(validator.InvariantsValidator):
... def validateObject(self, obj):
... errors = super(CustomValidator, self).validateObject(obj)
... if len(obj.email) > 2 * len(obj.login):
... errors += (zope.interface.Invalid('Email too long.'),)
... return errors

Since the ``validate()`` method of ``InvatiantsValidator`` simply uses
``validateObject()`` it is enough to only override ``validateObject()``. Now
we can use the validator:

>>> custom = CustomValidator(
... None, None, None, IPerson, None)

>>> custom.validate(
... {'login': u'srichter', 'email': u'[e-mail]'})
()
>>> custom.validate(
... {'login': u'srichter', 'email': u'[e-mail]'})
(<zope.interface.exceptions.Invalid instance at ...>,)

To register the custom validator only for this schema, we hace to use the
discriminator generator again.

>>> from z3c.form import util
>>> validator.WidgetsValidatorDiscriminators(
... CustomValidator, schema=util.getSpecification(IPerson, force=True))

Note: Of course we could have used the ``zope.component.adapts()`` function
from within the class, but I think it is too tediuos, since you have to
specify all discriminators and not only the specific ones you are
interested in.

After registering the validator,

>>> zope.component.provideAdapter(CustomValidator)

it becomes the validator for this schema:

>>> zope.component.queryMultiAdapter(
... (None, None, None, IPerson, None), interfaces.IManagerValidator)
<CustomValidator for IPerson>

>>> class ICar(zope.interface.Interface):
... pass
>>> zope.component.queryMultiAdapter(
... (None, None, None, ICar, None), interfaces.IManagerValidator)


The Data Wrapper
----------------

The ``Data`` class provides a wrapper to present a dictionary as a class
instance. This is used to check for invariants, which always expect an
object. While the common use cases of the data wrapper are well tested in the
code above, there are some corner cases that need to be addressed.

So let's start by creating a data object:

>>> data = validator.Data(IPerson, {'login': 'srichter', 'other': 1})

When we try to access a name that is not in the schema, we get an attribute
error:

>>> data.address
Traceback (most recent call last):
...
AttributeError: address

>>> data.other
Traceback (most recent call last):
...
AttributeError: other

If the field found is a method, then a runtime error is raised:

>>> class IExtendedPerson(IPerson):
... def compute():
... """Compute something."""

>>> data = validator.Data(IExtendedPerson, {'compute': 1})
>>> data.compute
Traceback (most recent call last):
...
RuntimeError: ('Data value is not a schema field', 'compute')

=======
Widgets
=======

Widgets are small UI components that accept and process the textual user
input. The only responsibility of a widget is to represent a value to the
user, allow it to be modified and then return a new value. Good examples of
widgets include the Qt widgets and HTML widgets. The widget is not responsible
for converting its value to the desired internal value or validate the
incoming data. These resposnibilities are passed data converters and
validators, respecitively.

There are several problems that can be identified in the original Zope 3 widget
implementation located at ``zope.app.form``.

(1) Field Dependence -- Widgets are always views of fields. While this might
be a correct choice for a high-level API, it is fundamentally wrong. It
disallows us to use widgets without defining fields. This also couples
certain pieces of information too tightly to the field, especially, value
retrieval from and storage to the context, validation and raw data
conversion.

(2) Form Dependence -- While widgets do not have to be located within a form,
they are usually tightly coupled to it. It is very difficult to use
widgets outside the context of a form.

(3) Traversability -- Widgets cannot be traversed, which means that they
cannot interact easily using Javascript. This is not a fundamental
problem, but simply a lack of the current design to recognize that small
UI components must also be traversable and thus have a URI.

(4) Customizability -- A consequence of issue (1) is that widgets are not
customizable enough. Implementing real-world projects has shown that
widgets often want a very fine-grained ability to customize values. A
prime example is the label. Because the label of a widget is retrieved
from the field title, it is impossible to provide an alternative label for
a widget. While the label could be changed from the form, this would
require rewriting the entire form to change a label. Instead, we often
endde up writing cusom schemas.

(5) Flexbility -- Oftentimes it is desired to have one widget, but multiple
styles of representation. For example, in one scenario the widget uses a
plain HTML widget and in another a fancy JavaScript widget is used. The
current implementation makes it very hard to provide alternative styles
for a widget.


Creating and Using Simple Widgets
---------------------------------

When using the widget API by itself, the simplest way to use it is to just
instantiate it using the request:

>>> from z3c.form.testing import TestRequest
>>> from z3c.form import widget
>>> request = TestRequest()
>>> age = widget.Widget(request)

In this case we instantiated a generic widget. A full set of simple
browser-based widgets can be found in the ``browser/`` package. Since no
helper components are around to fill the attributes of the widget, we have to
do it by hand:

>>> age.name = 'age'
>>> age.label = u'Age'
>>> age.value = '39'

The most important attributes are the "name" and the "value". The name is used
to identify the widget within the form. The value is either the value to be
manipulated or the default value. The value must be provided in the form the
widget needs it. It is the responsibility of a data converter to convert
between the widget value and the desired internal value.

Before we can render the widget, we have to register a template for the
widget. The first step is to define the template:

>>> import tempfile
>>> textWidgetTemplate = tempfile.mktemp('text.pt')
>>> open(textWidgetTemplate, 'w').write('''\
... <input type="text" name="" value=""
... tal:attributes="name view/name; value view/value;" />\
... ''')

Next, we have to create a template factory for the widget:

>>> from z3c.form.widget import WidgetTemplateFactory
>>> factory = WidgetTemplateFactory(
... textWidgetTemplate, widget=widget.Widget)

The first argument, which is also required, is the path to the template
file. An optional ``content_type`` keyword argument allows the developer to
specify the output content type, usually "text/html". Then there are five
keyword arguments that specify the discriminators of the template:

* ``context`` -- This is the context in which the widget is displayed. In a
simple widget like the one we have now, the context is ``None``.

* ``request`` -- This discriminator allows you to specify the type of request
for which the widget will be available. In our case this would be a browser
request. Note that browser requests can be further broken into layer, so you
could also specify a layer interface here.

* ``view`` -- This is the view from which the widget is used. The simple
widget at hand, does not have a view associated with it though.

* ``field`` -- This is the field for which the widget provides a
representation. Again, this simple widget does not use a field, so it is
``None``.

* ``widget`` -- This is the widget itself. With this discriminator you can
specify for which type of widget you are providing a template.

We can now register the template factory. The name of the factory is the mode
of the widget. By default, there are two widget modes: "input" and
"display". However, since the mode is just a string, one can develop other
kinds of modes as needed for a project. The default mode is "input":

>>> from z3c.form import interfaces
>>> age.mode is interfaces.INPUT_MODE
True

>>> import zope.component
>>> zope.component.provideAdapter(factory, name=interfaces.INPUT_MODE)

Once everything is set up, the widget is updated and then rendered:

>>> age.update()
>>> print age.render()
<input type="text" name="age" value="39" />

If a value is found in the request, it takes precedence, since the user
entered the value:

>>> age.request = TestRequest(form={'age': '25'})
>>> age.update()
>>> print age.render()
<input type="text" name="age" value="25" />

However, there is an option to turn off all request data:

>>> age.value = '39'
>>> age.ignoreRequest = True
>>> age.update()
>>> print age.render()
<input type="text" name="age" value="39" />


Creating and Using Field Widgets
--------------------------------

An extended form of the widget allows fields to control several of the
widget's properties. Let's create a field first:

>>> ageField = zope.schema.Int(
... __name__ = u'age',
... title = u'Age',
... min = 0,
... max = 130)

We can now use our simple widget and create a field widget from it:

>>> ageWidget = widget.FieldWidget(ageField, age)

Such a widget provides ``IFieldWidget``:

>>> interfaces.IFieldWidget.providedBy(ageWidget)
True

Of course, this is more commonly done using an adapter. Commonly those
adapters look like this:

>>> @zope.component.adapter(zope.schema.Int, TestRequest)
... @zope.interface.implementer(interfaces.IFieldWidget)
... def IntWidget(field, request):
... return widget.FieldWidget(field, widget.Widget(request))

>>> zope.component.provideAdapter(IntWidget)
>>> ageWidget = zope.component.getMultiAdapter((ageField, request),
... interfaces.IFieldWidget)

Now we just have to update and render the widget:

>>> ageWidget.update()
>>> print ageWidget.render()
<input type="text" name="age" />

There is no initial value for the widget, since there is no value in the
request and the field does not provide a default. Let's now give our field a
default value and see what happens:

>>> ageField.default = 30
>>> ageWidget.update()
Traceback (most recent call last):
...
TypeError: ('Could not adapt', <Widget u'age'>,
<InterfaceClass z3c.form.interfaces.IDataConverter>)

In order for the widget to be able to take the field's default value and use
it to provide an initial value the widget, we need to provide a data converter
that defines how to convert from the field value to the widget value.

>>> from z3c.form import converter
>>> zope.component.provideAdapter(converter.FieldWidgetDataConverter)
>>> zope.component.provideAdapter(converter.FieldDataConverter)

>>> ageWidget.update()
>>> print ageWidget.render()
<input type="text" name="age" value="30" />

Again, the request value is honored above everything else:

>>> ageWidget.request = TestRequest(form={'age': '25'})
>>> ageWidget.update()
>>> print ageWidget.render()
<input type="text" name="age" value="25" />


Creating and Using Context Widgets
----------------------------------

When widgets represent an attribute value of an object, then this object must
be set as the context of the widget:

>>> class Person(object):
... age = 45

>>> ageWidget.context = Person()
>>> zope.interface.alsoProvides(ageWidget, interfaces.IContextAware)

The result is that the context value takes over precendence over the default
value:

>>> ageWidget.request = TestRequest()
>>> ageWidget.update()
Traceback (most recent call last):
...
ComponentLookupError: ((...), <InterfaceClass ...IDataManager>, u'')

This call fails because the widget does not know how to extract the value from
the context. Registering a data manager for the widget does the trick:

>>> from z3c.form import datamanager
>>> zope.component.provideAdapter(datamanager.AttributeField)

>>> ageWidget.update()
>>> print ageWidget.render()
<input type="text" name="age" value="45" />

The context can be explicitely ignored, making the widget display the default
value again:

>>> ageWidget.ignoreContext = True
>>> ageWidget.update()
>>> print ageWidget.render()
<input type="text" name="age" value="30" />

Again, the request value is honored above everything else:

>>> ageWidget.request = TestRequest(form={'age': '25'})
>>> ageWidget.ignoreContext = False
>>> ageWidget.update()
>>> print ageWidget.render()
<input type="text" name="age" value="25" />


Dynamically Changing Attribute Values
-------------------------------------

Once widgets are used within a framework, it is very tedious to write Python
code to adjust certain attributes, even though hooks exist. The easiest way to
change those attribute values is actually to provide an adapter that provides
the custom value.

We can create a custom label for the age widget:

>>> AgeLabel = widget.StaticWidgetAttribute(
... u'Current Age',
... context=None, request=None, view=None, field=ageField, widget=None)

Clearly, this code deos not require us to touch the orginal form and widget
code, given that we have enough control over the selection. In the example
above, all the selection discriminators are listed for demonstration
purposes. Of course, the label in this case can be created as follows:

>>> AgeLabel = widget.StaticWidgetAttribute(u'Current Age', field=ageField)

Much better, isn't it? Initially the label is the title of the field:

>>> ageWidget.label
u'Age'

Let's now simply register the label as a named adapter; the name is the name
of the attribute to change:

>>> zope.component.provideAdapter(AgeLabel, name='label')

Asking the widget for the label now will return the newly registered label:

>>> ageWidget.update()
>>> ageWidget.label
u'Current Age'

Of course, simply setting the label or changing the label extraction via a
sub-class are other options you might want to consider. Furthermore, you
could also create a computed attribute value or implement your own component.

Overriding any other attribtue, such as ``required``, is done in the same
way. If any widget provides new attributes, they are also overridable this
way. For example, the selection widget defines a label for the option that no
value was selected. We often want to override this, because the German
translation sucks or the wording is often too generic.

Overriding the default value is somewhat special due to the complexity of
obtaining the value. So let's register one now:

>>> AgeDefault = widget.StaticWidgetAttribute(50, field=ageField)
>>> zope.component.provideAdapter(AgeDefault, name="default")

Let's now instantiate, update and render the widget to see the default value:

>>> ageWidget = zope.component.getMultiAdapter((ageField, request),
... interfaces.IFieldWidget)
>>> ageWidget.update()
>>> print ageWidget.render()
<input type="text" name="age" value="50" />


Sequence Widget
---------------

A common use case in user interfaces is to ask the user to select one or more
items from a set of options/choices. The ``widget`` module provides a basic
widget implementation to support this use case.

The options available for selections are known as terms. Initially, there are
no terms:

>>> request = TestRequest()
>>> seqWidget = widget.SequenceWidget(request)
>>> seqWidget.name = 'seq'

>>> seqWidget.terms is None
True

There are two ways terms can be added, either manually or via an
adapter. Those term objects must provide ``ITerms``. There is no simple
default implementation, so we have to provide one ourselves:

>>> from zope.schema import vocabulary
>>> class Terms(vocabulary.SimpleVocabulary):
... zope.interface.implements(interfaces.ITerms)
... def getValue(self, token):
... return self.getTermByToken(token).value

>>> terms = Terms(
... [Terms.createTerm(1, 'v1', u'Value 1'),
... Terms.createTerm(2, 'v2', u'Value 2'),
... Terms.createTerm(3, 'v3', u'Value 3')])
>>> seqWidget.terms = terms

Once the ``terms`` attribute is set, updating the widgets does not change the
terms:

>>> seqWidget.update()
>>> [term.value for term in seqWidget.terms]
[1, 2, 3]

The value of a sequence widget is a tuple/list of term tokens. When extracting
values from the request, the values must be valid tokens, otherwise the
default value is returned:

>>> seqWidget.request = TestRequest(form={'seq': ['v1']})
>>> seqWidget.extract()
['v1']

>>> seqWidget.request = TestRequest(form={'seq': ['v4']})
>>> seqWidget.extract()
<NOVALUE>

>>> seqWidget.request = TestRequest(form={'seq-empty-marker': '1'})
>>> seqWidget.extract()
[]

Since the value of the widget is a tuple of tokens, when displaying the
values, they have to be converted to the title of the term:

>>> seqWidget.value = ('v1', 'v2')
>>> seqWidget.displayValue
[u'Value 1', u'Value 2']

To demonstrate how the terms is automatically chosen by a widget, we should
instantiate a field widget. Let's do this with a choice field:

>>> seqField = zope.schema.Choice(
... title=u'Sequence Field',
... vocabulary=terms)

Let's now create the field widget:

>>> seqWidget = widget.FieldWidget(seqField, widget.SequenceWidget(request))
>>> seqWidget.terms

The terms should be available as soon as the widget is updated:

>>> seqWidget.update()
Traceback (most recent call last):
...
ComponentLookupError: ((...), <InterfaceClass ...ITerms>, u'')

This failed, because we did not register an adapter for the terms yet. After
the adapter is registered, everything should work as expected:

>>> from z3c.form import term
>>> zope.component.provideAdapter(term.ChoiceTerms)

>>> seqWidget.update()
>>> seqWidget.terms
<z3c.form.term.ChoiceTerms object at ...>

So that's it. Everything else is the same from then on.

===============
Action Managers
===============

Action managers are components that manage all actions that can be taken
within a view, usually a form. They are also responsible for executing actions
when asked to do so.

Creating an action manager
--------------------------

An action manager is a form-related adapter that has the following
discriminator: form, request, and content. While there is a base
implementation for an action manager, the ``action`` module does not provide a
full implementation.

So we first have to build a simple implementation based on the ``Actions``
manager base class which allows us to add actions. Note that the following
implementation is for deomnstration purposes. If you want to see a real action
manager implementation, then have a look at ``ButtonActions``. Let's now
implement our simple action manager:

>>> from z3c.form import action
>>> class SimpleActions(action.Actions):
... """Simple sample."""
...
... def append(self, name, action):
... """See z3c.form.interfaces.IActions."""
... if not name in self:
... self._data_keys.append(name)
... self._data_values.append(action)
... self._data[name] = action

Before we can initialise the action manager, we have to create instances for
our three discriminators, just enough to get it working:

>>> import zope.interface
>>> from z3c.form import interfaces
>>> class Form(object):
... zope.interface.implements(interfaces.IForm)
>>> form = Form()

>>> class Content(object):
... zope.interface.implements(zope.interface.Interface)
>>> content = Content()

>>> from z3c.form.testing import TestRequest
>>> request = TestRequest()

We are now ready to create the action manager, which is a simple
triple-adapter:

>>> manager = SimpleActions(form, request, content)
>>> manager
<SimpleActions None>

As we can see in the manager representation above, the name of the manager is
``None``, since we have not specified one:

>>> manager.__name__ = 'example'
>>> manager
<SimpleActions 'example'>


Managing and Accessing Actions
------------------------------

Initially there are no actions in the manager:

>>> manager.keys()
[]

Our simple implementation of has an additional ``append()`` method, which we
will use to add actions:

>>> apply = action.Action(request, u'Apply')
>>> manager.append(apply.name, apply)

The action is added immediately:

>>> manager.keys()
['apply']

However, you should not rely on it being added, and always update the manager
once all actions were defined:

>>> manager.update()

Note: If the title of the action is a more complex unicode string and no name
is specified for the action, then a hexadecimal name is created from the
title:

>>> action.Action(request, u'Apply Now!').name
'4170706c79204e6f7721'

Since the action manager is an enumerable mapping, ...

>>> from zope.interface.common.mapping import IEnumerableMapping
>>> IEnumerableMapping.providedBy(manager)
True

there are several API methods available:

>>> manager['apply']
<Action 'apply' u'Apply'>
>>> manager['foo']
Traceback (most recent call last):
...
KeyError: 'foo'

>>> manager.get('apply')
<Action 'apply' u'Apply'>
>>> manager.get('foo', 'default')
'default'

>>> 'apply' in manager
True
>>> 'foo' in manager
False

>>> manager.values()
[<Action 'apply' u'Apply'>]

>>> manager.items()
[('apply', <Action 'apply' u'Apply'>)]

>>> len(manager)
1


Executing actions
-----------------

When an action is executed, an execution adapter is looked up. If there is no
adapter, nothing happens. So let's create a request that submits the apply
button:

>>> request = TestRequest(form={'apply': 'Apply'})
>>> manager = SimpleActions(form, request, content)

We also want to have two buttons in this case, so that we can ensure that only
one is executed:

>>> apply = action.Action(request, u'Apply')
>>> manager.append(apply.name, apply)

>>> cancel = action.Action(request, u'Cancel')
>>> manager.append(cancel.name, cancel)
>>> manager.update()

Now that the manager is updated, we can ask it for the "executed" actions:

>>> manager.executedActions
[<Action 'apply' u'Apply'>]

Executing the actions does nothing, because there are no handlers yet:

>>> manager.execute()


Let's now register an action handler that listens to the "Apply" action. An
action handler has four discriminators: form, request, content, and
action. All those objects are available to the handler under those names. When
using the base action handler from the ``action`` module, ``__call__()`` is
the only method that needs to be implemented:

>>> from z3c.form import util

>>> class SimpleActionHandler(action.ActionHandlerBase):
... zope.component.adapts(
... None, TestRequest, None, util.getSpecification(apply))
... def __call__(self):
... print 'successfully applied'

>>> zope.component.provideAdapter(SimpleActionHandler)

As you can see, we registered the action specifically for the apply
action. Now, executing the actions calls this handler:

>>> manager.execute()
successfully applied

Of course it only works for the "Apply" action and not ""Cancel":

>>> request = TestRequest(form={'cancel': 'Cancel'})
>>> manager.request = apply.request = cancel.request = request
>>> manager.execute()

========================
Attribute Value Adapters
========================

In advanced, highly customized projects it is often the case that a property
wants to be overridden for a particular customer in a particular case. A prime
example is the label of a widget. Until this implementation of a form
framework was written, widgets only could get their label from the field they
were representing. Thus, wanting to change the label of a widget meant
implementing a custom schema and re-registering the form in question for the
custom schema. It is needless to say that this was very annoying.

For this form framework, we are providing multiple levels of customization.
The user has the choice to change the value of an attribute through attribute
assignment or adapter lookup. The chronological order of an attribute value
assignment is as follows:

1. During initialization or right thereafter, the attribute value can be set
by direct attribute assignment, i.e. ``obj.attr = value``

2. While updating the object, an adapter is looked up for the attribute. If an
adapter is found, the attribute value will be overridden. Of course, if the
object does not have an ``update()`` method, one can choose another
location to do the adapter lookup.

3. After updating, the developer again has the choice to override the attribute
allowing granularity above and beyond the adapter.

The purpose of this module is to implement the availability of an attribute
value using an adapter.

>>> from z3c.form import value

The module provides helper functions and classes, to create those adapters
with as little code as possible.


Static Value Adapter
--------------------

To demonstrate the static value adapter, let's go back to our widget label
example. Let's create a couple of simple widgets and forms first:

>>> class TextWidget(object):
... label = u'Text'
>>> tw = TextWidget()

>>> class CheckboxWidget(object):
... label = u'Checkbox'
>>> cbw = CheckboxWidget()

>>> class Form1(object):
... pass
>>> form1 = Form1()

>>> class Form2(object):
... pass
>>> form2 = Form2()

We can now create a generic widget property adapter:

>>> WidgetAttribute = value.StaticValueCreator(
... discriminators = ('widget', 'view')
... )

Creating the widget attribute object, using the helper function above, allows
us to define the discriminators (or the granulatrity) that can be used to
control a widget attribute by an adapter. In our case this is the widget
itself and the form/view in which the widget is displayed. In other words, it
will be possible to register a widget attribute value specifically for a
particular widget, a particular form, or a combination thereof.

Let's now create a label attribute adapter for the text widget, since our
customer does not like the default label:

>>> TextLabel = WidgetAttribute(u'My Text', widget=TextWidget)

The first argument of any static attribute value is the value itself, in our
case the string "My Text". The following keyword arguments are the
discriminators specified in the property factory. Since we only specify the
widget, the label will be available to all widgets. But first we have to
register the adapter:

>>> import zope.component
>>> zope.component.provideAdapter(TextLabel, name='label')

The name of the adapter is the attribute name of the widget. Let's now see how
we can get the label:

>>> from z3c.form import interfaces
>>> staticValue = zope.component.getMultiAdapter(
... (tw, form1), interfaces.IValue, name='label')
>>> staticValue
<StaticValue u'My Text'>

The resulting value object has one public method ``get()``, which returns the
actual value:

>>> staticValue.get()
u'My Text'

As we said before, the value should be available to all forms, ...

>>> zope.component.getMultiAdapter(
... (tw, form2), interfaces.IValue, name='label')
<StaticValue u'My Text'>

... but only to the ``TextWidget``:

>>> zope.component.getMultiAdapter(
... (cbw, form2), interfaces.IValue, name='label')
Traceback (most recent call last):
...
ComponentLookupError: ((<CheckboxWidget...>, <Form2...>),
<InterfaceClass ...IValue>, 'label')

By the way, the attribute adapter factory notices, if you specify a
discriminator that was not specified:

>>> WidgetAttribute(u'My Text', form=Form2)
Traceback (most recent call last):
...
ValueError: One or more keyword arguments did not match the discriminators.

>>> WidgetAttribute.discriminators
('widget', 'view')


Computed Value Adapter
----------------------

A second implementation of the value adapter in the evaluated value, where one
can specify a function that computes the value to be returned. The only
argument to the function is the value adapter instance itself, which then
contains all the discriminators as specified when creating the generic widget
attribute factory. Let's take the same use case as before, but generating the
value as follows:

>>> def getLabelValue(adapter):
... return adapter.widget.label + ' (1)'

Now we create the value adapter for it:

>>> WidgetAttribute = value.ComputedValueCreator(
... discriminators = ('widget', 'view')
... )

>>> TextLabel = WidgetAttribute(getLabelValue, widget=TextWidget)

After registering the adapter, ...

>>> zope.component.provideAdapter(TextLabel, name='label')

we now get the answers:

>>> from z3c.form import interfaces
>>> zope.component.getMultiAdapter(
... (tw, form1), interfaces.IValue, name='label')
<ComputedValue u'Text (1)'>


__Note__: The two implementations of the attribute value adapters are not
meant to be canonical features that must always be used. The API is
kept simple to allow you to quickly implement your own value
adapter.


Automatic Interface Assignment
------------------------------

Oftentimes it is desirable to register an attribute value adapter for an
instance. A good example is a field, so let's create a small schema:

>>> import zope.interface
>>> import zope.schema
>>> class IPerson(zope.interface.Interface):
... firstName = zope.schema.TextLine(title=u'First Name')
... lastName = zope.schema.TextLine(title=u'Last Name')

The customer now requires that the title -- which is the basis of the widget
label for field widgets -- of the last name should be "Surname". Until now the
option was to write a new schema changing the title. With this attribute value
module, as introduced thus far, we would need to provide a special interface
for the last name field, since registering a label adapter for all text fields
would also change the first name.

Before demonstrating the solution to this problem, let's first create a field
attribute value:

>>> FieldAttribute = value.StaticValueCreator(
... discriminators = ('field',)
... )

We can now create the last name title, changing only the title of the
``lastName`` field. Instead of passing in an interface of class as the field
discriminator, we pass in the field instance:

>>> LastNameTitle = FieldAttribute(u'Surname', field=IPerson['lastName'])

The attribute value factory will automatically detect instances, create an
interface on the fly, directly provide it on the field and makes it the
discriminator interface for the adapter registratioon.

So after registering the adapter, ...

>>> zope.component.provideAdapter(LastNameTitle, name='title')

the adapter is only available to the last name field and not the first name:

>>> zope.component.queryMultiAdapter(
... (IPerson['lastName'],), interfaces.IValue, name='title')
<StaticValue u'Surname'>

>>> zope.component.queryMultiAdapter(
... (IPerson['firstName'],), interfaces.IValue, name='title')

=============
Data Managers
=============

For the longest time the way widgets retrieved and stored their values on the
actual content/model was done by binding the field to a context and then
setting and getting the attribute from it. This has several distinct design
shortcomings:

1. The field has too much responsibility by knowing about its implementations.

2. There is no way of redefining the method used to store and access data
other than rewriting fields.

3. Finding the right content/model to modify is an implicit policy: Find an
adapter for the field's schema and then set the value there.

While implementing dome real-world projects, we noticed that this approach is
too limiting and we often could not use the form framework when we wanted or
had to jump through many hoops to make it work for us. For example, if we want
to display a form to collect data that does not correspond to a set of content
components, we was forced to not only write a schema for the form but also
implement that schema as a class. but all we wanted was a dictionary. For
edit-form like tasks we often also had an initial dictionary, which we just
wanted modified.

Data managers abstract the getting and setting of the data. A data manager is
responsible for setting one piece of data in a particular context.

>>> from z3c.form import datamanager


Attribute Field Manager
-----------------------

The most common case, of course, is the management of class attributes through
fields. In this case, the data manager needs to know about the context and the
field it is managing the data for.

>>> import zope.interface
>>> import zope.schema
>>> class IPerson(zope.interface.Interface):
... name = zope.schema.TextLine(
... title=u'Name',
... default=u'<no name>')

>>> class Person(object):
... zope.interface.implements(IPerson)
... name = u''
... def __init__(self, name):
... self.name = name

>>> stephan = Person(u'Stephan Richter')

We can now instantiate the data manager for Stephan's name:

>>> nameDm = datamanager.AttributeField(stephan, IPerson['name'])

The data manager consists of a few simple methods to accomplish its
purpose. Getting the value is done using the ``get()`` method:

>>> nameDm.get()
u'Stephan Richter'

The value can be set using ``set()``:

>>> nameDm.set(u'Stephan "Caveman" Richter')

>>> nameDm.get()
u'Stephan "Caveman" Richter'
>>> stephan.name
u'Stephan "Caveman" Richter'

A final feature that is supported by the data manager is the check whether a
value can be accessed and written. To demonstrate the feature, we first have
to provide security declarations for our person:

>>> from zope.security.management import endInteraction
>>> from zope.security.management import newInteraction
>>> from zope.security.management import setSecurityPolicy
>>> import z3c.form.testing
>>> endInteraction()
>>> newPolicy = z3c.form.testing.SimpleSecurityPolicy()
>>> newPolicy.allowedPermissions = ('View', 'Edit')
>>> oldpolicy = setSecurityPolicy(newPolicy)
>>> newInteraction()

>>> from zope.security.checker import Checker
>>> from zope.security.checker import defineChecker
>>> personChecker = Checker({'name':'View', 'name':'Edit'})
>>> defineChecker(Person, personChecker)

We now need to wrap stephan into a proxy:

>>> protectedStephan = zope.security.checker.ProxyFactory(stephan)

Since we are not logged in as anyone, we cannot acces or write the value:

>>> nameDm = datamanager.AttributeField(protectedStephan, IPerson['name'])

>>> nameDm.canAccess()
False
>>> nameDm.canWrite()
False

Clearly, this also means that ``get()`` and ``set()`` are also shut off:

>>> nameDm.get()
Traceback (most recent call last):
...
Unauthorized: (<Person object at ...>, 'name', 'Edit')

>>> nameDm.set(u'Stephan')
Traceback (most recent call last):
...
ForbiddenAttribute: ('name', <Person object at ...>)

Now we have to setup the security system and "log in" as a user:

>>> newPolicy.allowedPermissions = ('View', 'Edit')
>>> newPolicy.loggedIn = True

The created principal, with which we are logged in now, can only access the
attribute:

>>> nameDm.canAccess()
True
>>> nameDm.canWrite()
False

Thus only the ``get()`` method is allowed:

>>> nameDm.get()
u'Stephan "Caveman" Richter'

>>> nameDm.set(u'Stephan')
Traceback (most recent call last):
...
ForbiddenAttribute: ('name', <Person object at ...>)

If field's schema is not directly provided by the context, the datamanager
will attempt to find an adapter. Let's give the person an address for example:

>>> class IAddress(zope.interface.Interface):
... city = zope.schema.TextLine(title=u'City')

>>> class Address(object):
... zope.component.adapts(IPerson)
... zope.interface.implements(IAddress)
... def __init__(self, person):
... self.person = person
... @apply
... def city():
... def get(self):
... return getattr(self.person, '_city', None)
... def set(self, value):
... self.person._city = value
... return property(get, set)

>>> zope.component.provideAdapter(Address)

Now we can create a data manager for the city attribute:

>>> cityDm = datamanager.AttributeField(stephan, IAddress['city'])

Initially there is no value, but of course we can create one:

>>> cityDm.get()

>>> cityDm.set(u'Maynard')
>>> cityDm.get()
u'Maynard'

The value can be accessed through the adapter itself as well:

>>> IAddress(stephan).city
u'Maynard'

While we think that implicitly looking up an adapter is not the cleanest
solution, it allows us to mimic the behavior of ``zope.formlib``. We think
that we will eventually provide alternative ways to accomplish the same in a
more explicit way.

Finally, if we try to set a value that is read-only, a type error is raised:

>>> readOnlyName = zope.schema.TextLine(
... __name__='name',
... readonly=True)

>>> nameDm = datamanager.AttributeField(stephan, readOnlyName)
>>> nameDm.set(u'Stephan')
Traceback (most recent call last):
...
TypeError: Can't set values on read-only fields
(name=name, class=__builtin__.Person)


Dictionary Field Manager
------------------------

Another implementation of the data manager interface is provided by the
dictionary field manager, which does not expect an instance with attributes as
its context, but a dictionary. It still uses a field to determine the key to
modify.

>>> personDict = {}
>>> nameDm = datamanager.DictionaryField(personDict, IPerson['name'])

The datamanager can really only deal with dictionaries and no other types:

>>> datamanager.DictionaryField([], IPerson['name'])
Traceback (most recent call last):
...
ValueError: Data are not a dictionary: <type 'list'>

Let's now access the name:

>>> nameDm.get()
<NOVALUE>

Initially we get the default value (as specified in the field), since the
person dictionariy has no entry. If no default value has been specified in the
field, the missing value is returned.

Now we set a value and it should be available:

>>> nameDm.set(u'Roger Ineichen')

>>> nameDm.get()
u'Roger Ineichen'
>>> personDict
{'name': u'Roger Ineichen'}

Since this dictionary is not security proxied, any field can be accessed and
written to:

>>> nameDm.canAccess()
True
>>> nameDm.canWrite()
True

As with the attribute data manager, readonly fields cannot be set:

>>> nameDm = datamanager.DictionaryField(personDict, readOnlyName)
>>> nameDm.set(u'Stephan')
Traceback (most recent call last):
...
TypeError: Can't set values on read-only fields name=name


Cleanup
-------

We clean up the changes we made in these examples:

>>> endInteraction()
>>> ignore = setSecurityPolicy(oldpolicy)

==============
Data Converter
==============

The data converter is the component that converts an internal data value as
described by a field to an external value as required by a widget and vice
versa. The goal of the converter is to avoid field and widget proliferation
solely to handle different types of values. The purpose of fields is to
describe internal data types and structures and that of widgets to provide one
particular mean of input.

The only two discriminators for the converter are the field and the widget.

Let's look at the ``Int`` field to ``TextWidget`` converter as an example:

>>> import zope.schema
>>> age = zope.schema.Int(
... __name__='age',
... title=u'Age',
... min=0)

>>> from z3c.form.testing import TestRequest
>>> from z3c.form import widget
>>> text = widget.Widget(TestRequest())

>>> from z3c.form import converter
>>> conv = converter.FieldDataConverter(age, text)

The converter can now convert any integer to a the value the test widget deals
with, which is an ASCII string:

>>> conv.toWidgetValue(34)
u'34'

When the missing value is passed in, an empty string should be returned:

>>> conv.toWidgetValue(age.missing_value)
u''

Of course, values can also be converted from a widget value to field value:

>>> conv.toFieldValue('34')
34

Of course, trying to convert a non-integer string representation fails in a
conversion error:

>>> conv.toFieldValue('3.4')
Traceback (most recent call last):
...
ValueError: invalid literal for int(): 3.4

Also, the conversion to the field value also validates the data; in this case
negative values are not allowed:

>>> conv.toFieldValue('-34')
Traceback (most recent call last):
...
TooSmall: (-34, 0)

That's pretty much the entire API. When dealing with converters within the
component architecture, everything is a little bit simpler. So let's register
the converter:

>>> import zope.component
>>> zope.component.provideAdapter(converter.FieldDataConverter)

Once we ensure that our widget is a text widget, we can lookup the adapter:

>>> import zope.interface
>>> from z3c.form import interfaces
>>> zope.interface.alsoProvides(text, interfaces.ITextWidget)

>>> zope.component.getMultiAdapter((age, text), interfaces.IDataConverter)
<DataConverter from Int to Widget>

For field-widgets there is a helper adapter that makes the lookup even
simpler:

>>> zope.component.provideAdapter(converter.FieldWidgetDataConverter)

After converting our simple widget to a field widget,

>>> fieldtext = widget.FieldWidget(age, text)

we can now lookup the data converter adapter just by the field widget itself:

>>> interfaces.IDataConverter(fieldtext)
<DataConverter from Int to Widget>


Date Data Converter
-------------------

Since the ``Date`` field does not provide ``IFromUnicode``, we have to provide
a custom data converter. This default one is not very sophisticated and is
inteded for use with the text widget:

>>> date = zope.schema.Date()

>>> ddc = converter.DateDataConverter(date, text)
>>> ddc
<DataConverter from Date to Widget>

Dates are simply converted to ISO format:

>>> import datetime
>>> bday = datetime.date(1980, 1, 25)

>>> ddc.toWidgetValue(bday)
u'1980-01-25'

The converter only knows how to convert this particular format back to a
datetime value:

>>> ddc.toFieldValue(u'1980-01-25')
datetime.date(1980, 1, 25)


Time Data Converter
-------------------

Since the ``Time`` field does not provide ``IFromUnicode``, we have to provide
a custom data converter. This default one is not very sophisticated and is
inteded for use with the text widget:

>>> time = zope.schema.Time()

>>> tdc = converter.TimeDataConverter(time, text)
>>> tdc
<DataConverter from Time to Widget>

Dates are simply converted to ISO format:

>>> noon = datetime.time(12, 0, 0)

>>> tdc.toWidgetValue(noon)
u'12:00:00'

The converter only knows how to convert this particular format back to a
datetime value:

>>> tdc.toFieldValue(u'12:00:00')
datetime.time(12, 0)


Datetime Data Converter
-----------------------

Since the ``Datetime`` field does not provide ``IFromUnicode``, we have to
provide a custom data converter. This default one is not very sophisticated
and is inteded for use with the text widget:

>>> dtField = zope.schema.Datetime()

>>> dtdc = converter.DatetimeDataConverter(dtField, text)
>>> dtdc
<DataConverter from Datetime to Widget>

Dates are simply converted to ISO format:

>>> bdayNoon = datetime.datetime(1980, 1, 25, 12, 0, 0)

>>> dtdc.toWidgetValue(bdayNoon)
u'1980-01-25 12:00:00'

The converter only knows how to convert this particular format back to a
datetime value:

>>> dtdc.toFieldValue(u'1980-01-25 12:00:00')
datetime.datetime(1980, 1, 25, 12, 0)


Timedelta Data Converter
------------------------

Since the ``Timedelta`` field does not provide ``IFromUnicode``, we have to
provide a custom data converter. This default one is not very sophisticated
and is inteded for use with the text widget:

>>> timedelta = zope.schema.Timedelta()

>>> tddc = converter.TimedeltaDataConverter(timedelta, text)
>>> tddc
<DataConverter from Timedelta to Widget>

Dates are simply converted to ISO format:

>>> allOnes = datetime.timedelta(1, 3600+60+1)

>>> tddc.toWidgetValue(allOnes)
u'1 day, 1:01:01'

The converter only knows how to convert this particular format back to a
datetime value:

>>> tddc.toFieldValue(u'1 day, 1:01:01')
datetime.timedelta(1, 3661)


Sequence Data Converter
-----------------------

For widgets and fields that work with choices of a sequence, a special data
converter is required that works with terms. A prime example is a choice
field. Before we can use the converter, we have to register the terms adapter:

>>> from z3c.form import term
>>> zope.component.provideAdapter(term.ChoiceTerms)

Let's now create a choice field and a widget:

>>> from zope.schema.vocabulary import SimpleVocabulary

>>> gender = zope.schema.Choice(
... vocabulary = SimpleVocabulary([
... SimpleVocabulary.createTerm(0, 'm', u'male'),
... SimpleVocabulary.createTerm(1, 'f', u'female'),
... ]) )

>>> from z3c.form import widget
>>> seqWidget = widget.SequenceWidget(TestRequest())

We now use the field and widget to instantiate the converter:

>>> sdv = converter.SequenceDataConverter(gender, seqWidget)

We can now convert a real value to a widget value, which will be the term's
token:

>>> sdv.toWidgetValue(0)
['m']

The result is always a sequence, since sequence widgets only deal collections
of values. Of course, we can convert the widget value back to an internal
value:

>>> sdv.toFieldValue(['m'])
0

Sometimes a field is not required. In those cases, the value can be the
missing value of the field:

>>> gender.missing_value = 'missing'

>>> sdv.toWidgetValue(gender.missing_value)
['--NOVALUE--']

Similarly, if "no value" has been specified in the widget, the missing value
of the field is returned:

>>> sdv.toFieldValue([u'--NOVALUE--'])
'missing'

Collection Sequence Data Converter
----------------------------------

For widgets and fields that work with a sequence of choices, another data
converter is required that works with terms. A prime example is a list
field. Before we can use the converter, we have to register the terms adapter:

>>> from z3c.form import term
>>> zope.component.provideAdapter(term.CollectionTerms)

Let's now create a set field and a widget:

>>> genders = zope.schema.List(value_type=gender)
>>> seqWidget = widget.SequenceWidget(TestRequest())

We now use the field and widget to instantiate the converter:

>>> csdv = converter.CollectionSequenceDataConverter(genders, seqWidget)

We can now convert a real value to a widget value, which will be the term's
token:

>>> csdv.toWidgetValue([0])
['m']

The result is always a sequence, since sequence widgets only deal collections
of values. Of course, we can convert the widget value back to an internal
value:

>>> csdv.toFieldValue(['m'])
[0]

For some field, like the ``Set``, the collection type is a tuple. Sigh. In
these cases we use the last entry in the tuple as the type to use:

>>> genders = zope.schema.Set(value_type=gender)
>>> seqWidget = widget.SequenceWidget(TestRequest())

>>> csdv = converter.CollectionSequenceDataConverter(genders, seqWidget)

>>> csdv.toWidgetValue(set([0]))
['m']

>>> csdv.toFieldValue(['m'])
set([0])

=====
Terms
=====

Terms are used to provide choices for sequence widgets or any other construct
needing them. Since Zope 3 already has sources and vocabularies, the base
terms class simply builds on them.

Thus, let's create a vocabulary first:

>>> from zope.schema import vocabulary
>>> ratings = vocabulary.SimpleVocabulary([
... vocabulary.SimpleVocabulary.createTerm(0, '0', u'bad'),
... vocabulary.SimpleVocabulary.createTerm(1, '1', u'okay'),
... vocabulary.SimpleVocabulary.createTerm(2, '2', u'good')
... ])

Now we can create the terms object:

>>> from z3c.form import term
>>> terms = term.Terms()
>>> terms.terms = ratings

Getting a term from a given value is simple:

>>> terms.getTerm(0).title
u'bad'
>>> terms.getTerm(3)
Traceback (most recent call last):
...
LookupError: 3

When converting values from their Web representation back to the internal
representation, we have to be able to look up a term by its token:

>>> terms.getTermByToken('0').title
u'bad'
>>> terms.getTerm('3')
Traceback (most recent call last):
...
LookupError: 3

However, often we just want the value so asking for the value that is
represented by a token saves usually one line of code:

>>> terms.getValue('0')
0
>>> terms.getValue('3')
Traceback (most recent call last):
...
LookupError: 3

You can also iterate through all terms:

>>> [entry.title for entry in terms]
[u'bad', u'okay', u'good']

Or ask how many terms you have in the first place:

>>> len(terms)
3

Finally the API allows you to check whether a particular value is available in
the terms:

>>> 0 in terms
True
>>> 3 in terms
False

Now, there are several terms implementations that were designed for particular
fields. Within the framework, terms are used as adapters with the follwoing
discriminators: context, request, form, field, and widget.

The first terms implementation is for ``Choice`` fields:

>>> import zope.schema

>>> ratingField = zope.schema.Choice(
... title=u'Rating',
... vocabulary=ratings)

>>> terms = term.ChoiceTerms(None, None, None, ratingField, None)
>>> [entry.title for entry in terms]
[u'bad', u'okay', u'good']

A similar terms implementation exists for a ``Bool`` field:

>>> truthField = zope.schema.Bool()

>>> terms = term.BoolTerms(None, None, None, truthField, None)
>>> [entry.title for entry in terms]
[u'yes', u'no']

Finally, there is a terms adapter for all collections:

>>> ratingsField = zope.schema.List(
... title=u'Ratings',
... value_type=ratingField)

>>> terms = term.CollectionTerms(None, None, None, ratingsField, None)
>>> [entry.title for entry in terms]
[u'bad', u'okay', u'good']

=============================
Utility Functions and Classes
=============================

This file documents the utility functiona and classes that are otherwise not
tested.

>>> from z3c.form import util


``getWidgetById(form, id)`` Function
------------------------------------

Given a form and a widget id, this function extracts the widget for you. First
we need to create a properly developed form:

>>> import zope.interface
>>> import zope.schema

>>> class IPerson(zope.interface.Interface):
... name = zope.schema.TextLine(title=u'Name')

>>> from z3c.form import form, field
>>> class AddPerson(form.AddForm):
... fields = field.Fields(IPerson)

>>> from z3c.form import testing
>>> testing.setupFormDefaults()

>>> addPerson = AddPerson(None, testing.TestRequest())
>>> addPerson.update()

We can now ask for the widget:

>>> util.getWidgetById(addPerson, 'form.widgets.name')
<TextWidget 'form.widgets.name'>

The widget id can be split into a prefix and a widget name. The id must always
start with the correct prefix, otherwise a value error is raised:

>>> util.getWidgetById(addPerson, 'myform.widgets.name')
Traceback (most recent call last):
...
ValueError: Id 'myform.widgets.name' must start with prefix 'form.widgets.'

If the widget is not found but the prefix is correct, ``None`` is returned:

>>> util.getWidgetById(addPerson, 'form.widgets.myname')