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Python API for creating and handling forms

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The w20e.forms package provides a powerful API for creating and handling electronic forms. The package is loosely based on XForms concepts (and Socrates QE, a Java implementation). The package intends to provide a drop-in alternative for standard plone, pyramid and django solutions, but is useable in any framework (or without a framework…).

Core concepts

The core concepts are as follows (so you can quickly decide whether you like this approach or not):

A form is a container for/composition of four things:

  1. data

  2. model

  3. view

  4. submission

This clearly separates the data, the data’s properties (datatype, whether something is required or not, etc.) and the renderable part of the form. This is also the main difference between this API and other solutions (afaik…). Where the usual approach is to define a form based on a Schema or other data-centered notion, like so:

foo = String("id", "label", validator=somefunction)

the approach of w20e.forms is to define:

foo = Field("fid")
props = FieldProperties("pid", ["fid"], required=True, datatype=int)
ctrl = Input("cid", "label", bind="fid")

where the properties and control are bound to the variable. This enables controls in your form that are not bound to any data you wish to collect, sharing of properties, etc.

Another important difference is that the API provides a structured way of defining properties for data, instead of having to define your own validation. See section 1.2 for details.


The data holds the variables you wish to collect with this form. A variable simply has an id, and if you like a default value.


The model holds all properties for a given form, like readonly-ness, requiredness, datatyping for variables, relevance, etc. All these properties are calculated from expressions, in this case Python expressions, so requiredness is not just true or false, it can be calculated based on an expression that can include other variables in your data. All variables from the form data are available in expressions via the ‘data’ dict, so if variable ‘foo’ would be required if variable ‘bar’ was set to 666, this can be expressed like so in the properties bound to foo:

… required=”data[‘bar’] == 666” …

In general, all expressions are eval-ed to something true or false. The model offers the following properties:

  • required: is a variable required or not?

  • relevant: is the variable relevant? Like maiden name would be irrelevant when gender is male. In general, the related control/widget for irrelevant variables would not be shown.

  • readonly: can a person change the value of a variable?

  • calculate: in stead of letting a person set the value of a variable, the variable is calculated.

  • constraint: check whether the expression evaluates to True.

  • datatype: datatype for the variable, like int, string, or more complex variables.

Properties are bound to variables by a bind attribute. A set of properties can be bound to a series of variables.


The view (or FormView) is the actual visible part (or audible for that matter) of the form. The view can be rendered and holds a collection of widgets or controls, that are bound to variables. More than one control can bind to the same variable. Controls can be grouped in groups for layout purposes, like flow layout or card layout (tabs).

In label and hint texts of controls you can use lexical values of variables by using the expression ${<var name>}. This way you can refer to values given in other variables from your labels and hints.

Basic use

Ok, enough theory, let’s do something for real.

A form is produced by hand, or by using a factory: this should take care of producing a form holding the necessary stuff.

Let’s get the imports over with…

>>> import sys
>>> from interfaces import *
>>> from zope.interface import implements
>>> from formdata import FormData
>>> from formview import FormView
>>> from formmodel import FormModel
>>> from data.field import Field
>>> from model.fieldproperties import FieldProperties
>>> from rendering.control import Input, Select, Option
>>> from import FlowGroup
>>> from form import Form, FormValidationError
>>> from rendering.html.renderer import HTMLRenderer
>>> from submission.attrstorage import AttrStorage

Creating a form

Now let us create a factory class

>>> class FormFactory():
...   implements(IFormFactory)
...   def createForm(self):
...     data = FormData()
...     data.addField(Field("field0"))
...     data.addField(Field("field1", "foo"))
...     data.addField(Field("field2", "bar"))
...     data.addField(Field("field3"))
...     view = FormView()
...     grp = FlowGroup("grp0", label="Group 0")
...     grp.addRenderable(Input("input2", "Input 2", bind="field0"))
...     view.addRenderable(Input("input0", "First name", bind="field0"))
...     view.addRenderable(Input("input1", "Last name", bind="field1"))
...     view.addRenderable(Select("select0", "Select me!", options=[], bind="field2", with_empty=True))
...     view.addRenderable(grp)
...     model = FormModel()
...     model.addFieldProperties(FieldProperties("prop0", ["field0"], required="True"))
...     model.addFieldProperties(FieldProperties("prop1", ["field1", "field2"], relevant="data['field0']"))
...     submission = AttrStorage(attr_name="_data")
...     return Form("test", data, model, view, submission)
>>> ff = FormFactory()
>>> form = ff.createForm()

By now, we should have a form where field0 is required, and field1 and field2 are only relevant if field0 is filled in.

>>> print len(
>>> props = form.model.getFieldProperties("field0")
>>> props[0].id
>>> len(props)
>>> field0 ="field0")
>>> field0.value

In the meanwhile, field1 and field2 should be irrelevant, given that field0 has no value

>>> form.model.isRelevant("field1",
>>> form.model.isRelevant("field2",

Validation should fail, given that field0 is required.

>>> try:
...   form.validate()
... except FormValidationError:
...   print sys.exc_info()[1].errors['field0']
>>>"field0").value = "pipo"
>>> form.validate()
>>> field0.value

By now, field1 and field2 should also be relevant

>>> form.model.isRelevant("field1",
>>> form.model.isRelevant("field2",


The following section will assume rendering to HTML. This will most likely cover nigh 100% of the use cases… Now for some display parts. An irrelevant control should not have a class ‘relevant’, otherwise it should have it… This enables specific styling, like ‘display: none’.

>>>'field0').value = None
>>> field = form.view.getRenderable('input1')
>>> renderer = HTMLRenderer()
>>> renderer.render(form, field, sys.stdout)
<div id="input1" class="control input ">
<label for="input-input1">Last name</label>
<div class="alert"></div>
<div class="hint"></div>
<input id="input-input1" type="text" name="input1" value="foo" size="20"/>
>>>'field0').value = 'pipo'
>>> field = form.view.getRenderable('input1')
>>> renderer = HTMLRenderer()
>>> renderer.render(form, field, sys.stdout)
<div id="input1" class="control input relevant">
<label for="input-input1">Last name</label>
<div class="alert"></div>
<div class="hint"></div>
<input id="input-input1" type="text" name="input1" value="foo" size="20"/>
>>> field = form.view.getRenderable('input0')
>>> renderer.render(form, field, sys.stdout)
<div id="input0" class="control input relevant required">
<label for="input-input0">First name</label>
<div class="alert"></div>
<div class="hint"></div>
<input id="input-input0" type="text" name="input0" value="pipo" size="20"/>

How ‘bout those extra classes…

>>> renderer.render(form, field, sys.stdout, extra_classes="card")
<div id="input0" class="control input card relevant required">
<label for="input-input0">First name</label>
<div class="alert"></div>
<div class="hint"></div>
<input id="input-input0" type="text" name="input0" value="pipo" size="20"/>
>>> select = form.view.getRenderable('select0')
>>> renderer.render(form, select, sys.stdout)
<div id="select0" class="control select relevant">
<label for="input-select0">Select me!</label>
<div class="alert"></div>
<div class="hint"></div>
<select id="input-select0" name="select0"  size="1">
<option value="" >Maak een keuze</option>

Do we actually get grouped controls?

>>> nested_input = form.view.getRenderable('input2')


Finally when the form is rendered, filled in by someone, and validated, the data should normally go somewhere. This is by way of submission. We defined submission to be AttrStorage, something that stores the data in an attribute on some context. This is a case that could be used in many frameworks, at least plone and pyramid.

Let’s see what it does:

>>> class Context:
...   """ some context """
>>> ctx = Context()
>>> form.submission.submit(form, ctx)

The context now should hold the data in an attribute. We specified the name of the attribute to be ‘_data’, so let’s check:

>>> ctx._data.getField('field0').value

Beyond the basics

Well, this is all very simple, and it is quite likely that you would wish for something a bit more usefull. All parts of the form are there to be extended. Take for instance the FormView. A developer (or end user) should be able to:

  • create a full HTML form;

  • use a generated HTML form (this is wat the base implementation does);

  • create a PDF form.

The factory is also an important part of the form process. A factory can be imagined to be one of the following:

  • produced from a Schema (content type);

  • produced from an XML definition, for example an XForms instance from OpenOffice.

Forms in general should be:

  • submitable to a range of handlers, like email, database storage, content type storage;

  • easy to validate ‘live;

  • enable multi-page.

More detailed tests:

We’d like to check whether lookup of a control by bind works, so as to be able to process values into lexical values. This is especially interesting when using selects: we’d expect to see the label not the value in lexical space.

>>> data = FormData()
>>> data.addField(Field("f0", "opt0"))
>>> view = FormView()
>>> opts = [Option("opt0", "Option 0"), Option("opt1", "Option 1")]
>>> view.addRenderable(Select("sel0", "Select 0", bind="f0", options=opts))
>>> ctl = view.getRenderableByBind("f0")
>>> ctl.lexVal("opt0")
'Option 0'

Can we use variable substitution in labels and hints? Yes, we can!

>>> data = FormData()
>>> data.addField(Field("f0", "Pipo"))
>>> data.addField(Field("f1"))
>>> view = FormView()
>>> view.addRenderable(Input("in0", "First name", bind="f0"))
>>> view.addRenderable(Input("in1", "Last name for ${f0}", bind="f1"))
>>> model = FormModel()
>>> form = Form("test", data, model, view, None)
>>> renderer = HTMLRenderer()
>>> field = form.view.getRenderable('in1')
>>> renderer.render(form, field, sys.stdout)
<div id="in1" class="control input relevant">
<label for="input-in1">Last name for Pipo</label>
<div class="alert"></div>
<div class="hint"></div>
<input id="input-in1" type="text" name="in1" value="" size="20"/>

Let’s delve into input processing a bit… A simple input should just return it’s own value

>>> data = {'pipo': 'lala'}
>>> ctl = Input("pipo", "f0", "Some input")
>>> ctl.processInput(data)

Registering your own stuff

w20e.forms is not a complete library for forms, and it will never be this, since most people have very specific needs, like a specific widget, a custom version of an input field, etc. The API facilitates in this by using a global registry to register extensions.

The global registry is available like so:

>>> from w20e.forms.registry import Registry

and offers a number of class methods to register stuff.

Let’s for exampe register a new renderer for an input:


w20e.forms enables use of vocabularies to limit possible answers to a given list. This is a feature that is generally used with select widgets. A vocabulary is a ‘named’ factory that creates a list of options.

Register like so:

>>> def make_vocab():
...   return [Option('0', 'Opt 0'), Option('1', 'Opt 1')]
... Registry.register_vocab('foovocab', make_vocab)
... sel = Select("select0", "Select me!", vocab=make_vocab,
...   bind="field2", with_empty=True))

Required, Relevant, Readonly

In a form you’ll usually want to say things like: this control need only be shown whan the answer to that question is ‘x’, or that question is required whenever the answer to somethind else is ‘y’.

w20e.forms enables this using expressions. The epxressions are set as properties in variables, by their ‘bind’ attribute. So in the form model you may have a property set named ‘req’, that makes variable ‘foo’ required like so:

model.addFieldProperties(FieldProperties(“req”, [“foo”], required=”True”))

Obviously in general you want something a bit more flexible than that, like checking for other data that has been entered. All form data is made available to the expression within the ‘data’ variable, that is a dict. So checking upon some other variable, goes like this:

model.addFieldProperties(FieldProperties(“req”, [“foo”],

required=”data[‘bar’] == 42”))

So only if the answer to ‘bar’ is 42, ‘foo’ is required. Relevance, requiredness and readonly-ness all work like this.

You may even add your own expression context to the engine, to call methods on objects, etc.

Go like this, assuming your object is obj:

>>> registry.register_expr_context('mycontext', obj)
... model.addFieldProperties(FieldProperties("req", ["foo"],
...   relevant="mycontext.some_method())


The xml namespace of the w20e.forms package provides an XML based implementation of the w20e.forms API. This enables definition from and serialization to XML files. Provided is the DTD used for defining the w20e.forms as XML. This is quite similar to xForms.

Using XML as definition of forms provides a more declarative way of creating forms, not unlike the way you create a form in HTML. Also, XML is a format that is easily stored and transported.

Start using the XML factory

>>> from factory import XMLFormFactory

Now let us create a factory class

>>> xml = """
... <form id="test">
...   <!-- The data part, a.k.a. the variables you wish to collect -->
...   <data>
...     <foo/>
...     <bar value="666"/>
...   </data>
...   <model>
...     <properties id="required">
...       <bind>foo</bind>
...       <bind>bar</bind>
...       <required>True</required>
...     </properties>
...     <properties id="int">
...       <bind>bar</bind>
...       <datatype>int</datatype>
...     </properties>
...   </model>
...   <view>
...     <input id="fooctl" bind="foo">
...       <label>Foo?</label>
...       <hint>Well, foo or no?</hint>
...     </input>
...     <select id="barctl" bind="bar">
...       <property name="multiple">False</property>
...       <label>Bar</label>
...       <option value="1">One</option>
...       <option value="2">Two</option>
...     </select>
...     <select bind="bar" id="barctl2">
...       <label>Bar2</label>
...       <option value="3">Three</option>
...       <option value="4">Four</option>
...     </select>
...     <select bind="bar" id="barctl3">
...       <property name="vocab">some_vocab</property>
...       <label>Bar3</label>
...     </select>
...     <group layout="flow" id="groupie">
...       <label>GruppoSportivo</label>
...       <text id="txt">Moi</text>
...     </group>
...   </view>
...   <submission type="none">
...     <property name="action">@@save</property>
...   </submission>
... </form>"""

We are using a vocab in the xml, so register it… >>> from w20e.forms.registry import Registry … def some_vocab(): … return [Option(0, 0), Option(1, 1)] … Registry.register_vocab(‘some_vocab’, some_vocab)

>>> xmlff = XMLFormFactory(xml)
>>> form = xmlff.create_form()
>>> print len(
>>> print"foo").id
>>> print"bar").value

Set the value

>>>"bar").value = 777
>>> print"bar").value

Okido, so far so good. Now let’s see what properties we have.

>>> props = form.model.getFieldProperties("bar")
>>> len(props)
>>> intprop = [prop for prop in props if == "int"][0]
>>> reqprop = [prop for prop in props if == "required"][0]
>>> reqprop.getRequired()
>>> intprop.getDatatype()

Finally, check the viewable part, or the controls >>> ctrl = form.view.getRenderable(“fooctl”) >>> ctrl.label ‘Foo?’

>>> ctrl.__class__.__name__
>>> ctrl.hint
'Well, foo or no?'
>>> ctrl.bind
>>> ctrl = form.view.getRenderable("barctl")
>>> ctrl.multiple
>>> len(ctrl.options)
Do we get the nested stuff?
>>> ctrl = form.view.getRenderable("txt")


You can easily serilialize the form back into XML. Let’s try…

>>> from serializer import XMLSerializer
>>> serializer = XMLSerializer()
>>> print serializer.serialize(form)
<form id="test">
    <bar value="777"/>
    <properties id="int">
    <properties id="required">
    <input bind="foo" id="fooctl">
      <hint>Well, foo or no?</hint>
    <select bind="bar" id="barctl">
      <property name="multiple">False</property>
      <option value="1">One</option>
      <option value="2">Two</option>
    <select bind="bar" id="barctl2">
      <option value="3">Three</option>
      <option value="4">Four</option>
    <flowgroup id="groupie">
      <text id="txt"/>
  <submission type="none">
    <property name="action">@@save</property>

Note that variable foo now holds the value 777. Sadly, it is hard to guarantee that all XML will be exactely the same as the input XML.


The pyramid package provides a simple means of using w20e.forms for pyramid apps. The package provides a specific ‘file’ field for pyramid, to enable extracting filename and contents from a file in a POST/GET request, and a base view.

Would you wish to use w20e.forms, then:

  • add w20e.forms to the eggs dependencies of your app (duh…)

  • for the view that you wish to show the actual form, override w20e.forms.pyramid.pyramidformview. Let’s do some imports first. Please note that it is more convenient to use the XML implementation, as shown later on. Also, if you insist on using the Pythonic implementation, it is better to make a factory create the form, so you can just call the factory from your view. Anyway, let’s go for the not-so-smart way:

    >>> from w20e.forms.form import Form
    >>> from w20e.forms.formdata import FormData
    >>> from w20e.forms.formmodel import FormModel
    >>> from w20e.forms.formview import FormView
    >>> from w20e.forms.submission.attrstorage import AttrStorage
    >>> from import Field
    >>> from w20e.forms.rendering.control import Input
    >>> from w20e.forms.pyramid.formview import formview as pyramidformview

    Phew, that was a load of imports. Now do the actual view class. It’s a pretty simple form, but you should get the picture.

    >>> class yourformview(pyramidformview):
    ...   def __init__(self, context, request):
    ...     data = FormData()
    ...     data.addField(Field("foo", "some default value"))
    ...     data.addField(Field("bar"))
    ...     model = FormModel()
    ...     view = FormView()
    ...     # We'll leave the poperties out for now, check the main
    ...     # README for details
    ...     view.addRenderable(Input("input0", "Input foo", bind="foo"))
    ...     view.addRenderable(Input("input1", "Input bar here", bind="bar"))
    ...     submission = AttrStorage(attr_name="_data")
    ...     form = Form("test", data, model, view, submission)
    ...     pyramidformview.__init__(self, context, request, form)

    Now, a view for pyramid just takes a context, and a request, so let’s create the view instance:

    >>> class Context:
    ...  """ nothing needed here, but we'll store the data in here """
    >>> class Request:
    ...   def __init__(self, params=None):
    ...     self.params = params
    >>> ctx = Context()
    >>> req = Request()
    >>> view = yourformview(ctx, req)

    Ok, we’re ready for some action now. Let’s try to render the form.

    >>> print view.renderform()
    <form class="w20e-form" method="post" action="" enctype="multipart/form-data">
    <input type="hidden" name="formprocess" value="1"/>
    <div class="alert"></div>
    <div id="input0" class="control input relevant">
    <label for="input-input0">Input foo</label>
    <div class="alert"></div>
    <div class="hint"></div>
    <input id="input-input0" type="text" name="input0" value="some default value" size="20"/>
    <div id="input1" class="control input relevant">
    <label for="input-input1">Input bar here</label>
    <div class="alert"></div>
    <div class="hint"></div>
    <input id="input-input1" type="text" name="input1" value="" size="20"/>

    Nice. Now let’s give the request some content, and let the view handle the submission. This should result in the context having the form data stored in the _data attribute. formprocess is the marker used by w20e.forms to assume that the form is posted.

    >>> req = Request({'formprocess': 1, 'input0': 6, 'input1': 'whatever'})
    >>> view = yourformview(ctx, req)
    >>> view()
    {'status': 'stored', 'errors': {}}
    >>> ctx._data.getField('foo').value
    >>> ctx._data.getField('bar').value

XML implementation

Using the XML implementation makes life even easier:

from w20e.forms.pyramid.formview import xmlformview as pyramidformview
from w20e.forms.xml.formfile import FormFile

class yourformview(pyramidformview):

  def __init__(self, context, request):
      pyramidformview.__init__(self, context, request, FormFile("forms/yourform.xml"))

where you have a directory ‘forms’ containing the XML definition called yourform.xml. Check the w20e.forms.xml module for details on XML definitions.

  • Create a template ( for example) that calls the render method of the view:

    <p tal:content="structure python:view.renderform()"></p>
  • Wire the stuff into zcml (assuming you use that), like so:


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