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A debconf-like (or about:config-like) registry for storing application settings

Project description


This package provides debconf-like (or about:config-like) settings registries
for Zope applications. A `registry`, with a dict-like API, is used to get and
set values stored in `records`. Each record contains the actual value, as
well as a `field` that describes the record in more detail. At a minimum, the
field contains information about the type of value allowed, as well as a short
title describing the record's purpose.

See the following doctests for more details:

* `registry.txt`, which demonstrates how registries and records work
* `events.txt`, which shows the events that are fired from the registry
* `field.txt`, which describes the behaviour of persistent fields


1.0a1 - 2009-04-17

* Initial release

Using registries

You can create a new registry simply by instantiating the Registry class. The
class and its data structures are persistent, so you can store them in the
ZODB. You may want to provide the registry object as local utility for easy
access as well, though we won't do that here.

>>> from plone.registry import Registry
>>> registry = Registry()

The registry starts out empty. To access the registry's records, you can use
the `records` property. This exposes a dict API where keys are strings and
values are objects providing `IRecords`.

>>> len(registry.records)

Simple records

Let's now create a record. A record must have a name. This should be a dotted
name, and contain ASCII characters only. By convention, it should be all
lowercase and start with the name of the package that defines the record.

It is also possible to create a number of records based on a single schema
interface - see below - but for now, we will focus on simple records.

Before we can create the record, we must create the field that describes it.
Fields are based on the venerable zope.schema package, but plone.registry
only supports certain fields, and disallows use of a few properties even
of those. As a rule of thumb, so long as a field stores a Python primitive,
it is supported; the same goes for attributes of fields.


* Fields like `Object`, `InterfaceField` and so on are not supported
* A custom `constraint` method is not supported.
* The `order` attribute will always be set to -1
* For Choice fields, only named vocabularies are supported: you cannot
reference a particular source or source binder
* The key_type and value_type properties of Dict, List, Tuple, Set and
Frozenset may only contain persistent fields.

See field.text for more details.

Creating a record

The supported field types are found in the module plone.registry.field. These
are named the same as the equivalent field in zope.schema, and have the same
constructors. You must use one of these fields when creating records directly.

>>> from plone.registry import field
>>> age_field = field.Int(title=u"Age", min=0, default=18)

>>> from plone.registry import Record
>>> age_record = Record(age_field)

Note that in this case, we did not supply a value. The value will therefore
be the field default.

>>> age_record.value

We can set a different value, either in the `Record` constructor or via the
`value` attribute:

>>> age_record.value = 2
>>> age_record.value

Note that the value is validated against the field:

>>> age_record.value = -1
Traceback (most recent call last):
TooSmall: (-1, 0)

>>> age_record.value

We can now add the field to the registry. This is done via the `record`

>>> 'plone.records.tests.age' in registry
>>> registry.records['plone.records.tests.age'] = age_record

At this point, the record will gain __name__ and __parent__ attributes.

>>> age_record.__name__

>>> age_record.__parent__ is registry

Creating a record with an initial value

We can create records more succinctly by creating the field, setting the value
and assigning it to the registry in one go, like this:

>>> registry.records['plone.records.tests.cms'] = \
... Record(field.TextLine(title=u"CMS of choice"), u"Plone")

The record can now be obtained. Note that it has a nice __repr__ to help

>>> registry.records['plone.records.tests.cms']
<Record plone.records.tests.cms>

Accessing and manipulating record values

Once a record has been created and added to the registry, you can access
its value through dict-like operations on the registry itself.

>>> 'plone.records.tests.cms' in registry

>>> registry['plone.records.tests.cms']

>>> registry['plone.records.tests.cms'] = u"Plone 3.x"

Again, values are validated:

>>> registry['plone.records.tests.cms'] = 'Joomla'
Traceback (most recent call last):
WrongType: ('Joomla', <type 'unicode'>)

There is also a get() method:

>>> registry.get('plone.records.tests.cms')
u'Plone 3.x'
>>> registry.get('non-existent-key') is None

Deleting records

Records may be deleted from the `records` property:

>>> del registry.records['plone.records.tests.cms']
>>> 'plone.records.tests.cms' in registry.records
>>> 'plone.records.tests.cms' in registry

Creating records from interfaces

As an application developer, it is often desirable to define settings as
traditional interfaces with zope.schema fields. plone.registry includes
support for creating a set of records from a single interface.

To test this, we have created an interface, `IMailSettings`, with two fields,
`sender` and `smtp_host`.

>>> from plone.registry.tests import IMailSettings

Note that this contains standard fields.

>>> IMailSettings['sender']
<zope.schema._bootstrapfields.TextLine object at ...>

>>> IMailSettings['smtp_host']
<zope.schema._field.URI object at ...>

We can create records from this interface like this:

>>> registry.register_interface(IMailSettings)

One record for each field in the interface has now been created. Their names
are the full dotted names to those fields:

>>> sender_record = registry.records['plone.registry.tests.IMailSettings.sender']
>>> smtp_host_record = registry.records['plone.registry.tests.IMailSettings.smtp_host']

The fields used in the records will be the equivalent persistent versions of
the fields from the original interface.

>>> sender_record.field
<plone.registry.field.TextLine object at ...>

>>> smtp_host_record.field
<plone.registry.field.URI object at ...>

This feat is accomplished internally by adapting the field to the
IPersistentField interface. There is a default adapter factory that works for
all fields defined in plone.registry.field. You can of course define your own
adapter if you have a custom field type, but bear in mind the golden rules of
any persistent field:

* The field must store only primitives or other persistent fields
* It must not reference a function, class, interface or other method that
could break if a package is uninstalled.

If we have a field for which there is no IPersistentField adapter, we will
get an error:

>>> from plone.registry.tests import IMailPreferences
>>> IMailPreferences['settings']
<zope.schema._field.Object object at ...>

>>> registry.register_interface(IMailPreferences)
Traceback (most recent call last):
TypeError: There is no persistent field equivalent for the field `settings` of type `Object`.

Whoops! We can, however, tell register_interface() to ignore one or more

>>> registry.register_interface(IMailPreferences, omit=('settings',))

Once an interface's records have been registered, we can get and set their
values as normal:

>>> registry['plone.registry.tests.IMailSettings.sender']

>>> registry['plone.registry.tests.IMailSettings.sender'] = u"webmaster@localhost"
>>> registry['plone.registry.tests.IMailSettings.sender']

If we sub-sequently re-register the same interface, the value will be retained
if possible:

>>> registry.register_interface(IMailSettings)
>>> registry['plone.registry.tests.IMailSettings.sender']

However, if the value is no longer valid, we will revert to the default. To
test that, let's sneakily modify the field for a while.

>>> old_field = IMailSettings['sender']
>>> IMailSettings._InterfaceClass__attrs['sender'] = field.Int(title=u"Definitely not a string", default=2)
>>> if hasattr(IMailSettings, '_v_attrs'):
... del IMailSettings._v_attrs['sender']
>>> registry.register_interface(IMailSettings)
>>> registry['plone.registry.tests.IMailSettings.sender']

But let's put it back the way it was.

>>> IMailSettings._InterfaceClass__attrs['sender'] = old_field
>>> if hasattr(IMailSettings, '_v_attrs'):
... del IMailSettings._v_attrs['sender']
>>> registry.register_interface(IMailSettings)
>>> registry['plone.registry.tests.IMailSettings.sender']

Accessing the original interface

Now that we have these records, we can look up the original interface. This
does not break the golden rules: internally, we only store the name of the
interface, and resolve it at runtime.

Records that know about interfaces are marked with `IInterfaceAwareRecord` and
have two additional properties: `interface` and `field_name`.

>>> from plone.registry.interfaces import IInterfaceAwareRecord
>>> IInterfaceAwareRecord.providedBy(age_record)
>>> IInterfaceAwareRecord.providedBy(sender_record)

>>> sender_record.interface_name

>>> sender_record.interface is IMailSettings

Using the records proxy

Once the records for an interface has been created, it is possible to obtain
a proxy object that provides the given interface, but reads and writes its
values to the registry. This is useful, for example, to create a form using
`zope.formlib` or `z3c.form` that is configured with widgets based on the
interface, or simply as a more convenient API when working with multiple,
related settings.

>>> proxy = registry.for_interface(IMailSettings)
>>> proxy
<RecordsProxy for plone.registry.tests.IMailSettings>

The proxy is not a persistent object on its own.

>>> from persistent.interfaces import IPersistent
>>> IPersistent.providedBy(proxy)

It does, however, provide the requisite interface.

>>> IMailSettings.providedBy(proxy)

You can distinguish between the proxy and a 'norma' object by checking for the
IRecordsProxy marker interface:

>>> from plone.registry.interfaces import IRecordsProxy
>>> IRecordsProxy.providedBy(proxy)

When we set a value, it is stored in the registry:

>>> proxy.smtp_host = ''
>>> registry['plone.registry.tests.IMailSettings.smtp_host']

>>> registry['plone.registry.tests.IMailSettings.smtp_host'] = 'smtp://'
>>> proxy.smtp_host

Values not in the interface will raise an AttributeError:

>>> proxy.age
Traceback (most recent call last):
AttributeError: age

Note that by default, the for_interface() method will check that the necessary
records have been registered. For example, we cannot use any old interface:

>>> registry.for_interface(IInterfaceAwareRecord)
Traceback (most recent call last):
KeyError: 'Interface `plone.registry.interfaces.IInterfaceAwareRecord` defines a field `interface`, for which there is no record.'

By default, we also cannot use an interface for which only some records exist:

>>> registry.for_interface(IMailPreferences)
Traceback (most recent call last):
KeyError: 'Interface `plone.registry.tests.IMailPreferences` defines a field `settings`, for which there is no record.'

It is possible to disable this check, however. This will be a bit more

>>> registry.for_interface(IMailPreferences, check=False)
<RecordsProxy for plone.registry.tests.IMailPreferences>

A better way, however, is to explicitly declare that some fields are omitted:

>>> pref_proxy = registry.for_interface(IMailPreferences, omit=('settings',))

In this case, the omitted fields will default to their 'missing' value:

>>> pref_proxy.settings == IMailPreferences['settings'].missing_value

However, trying to set the value will result in a AttributeError:

>>> pref_proxy.settings = None
Traceback (most recent call last):
AttributeError: settings===============
Registry events

The registry fires certain events. These are:

* `plone.registry.interfaces.IRecordAddedEvent`, when a record has been
added to the registry.

* `plone.registry.interfaces.IRecordRemovedEvent`, when a record has been
removed from the registry.

* `plone.registry.interfaces.IRecordModifiedEvent`, when a record's value is

To test these events, we will create, modify and remove a few records:

>>> from plone.registry import Registry, Record, field
>>> registry = Registry()

Adding a new record to the registry should fire IRecordAddedEvents:

>>> registry.records['plone.registry.tests.age'] = \
... Record(field.Int(title=u"Age", min=0, default=18))

>>> registry.records['plone.registry.tests.cms'] = \
... Record(field.TextLine(title=u"Preferred CMS"), value=u"Plone")

When creating records from an interface, one event is fired for each field
in the interface.

>>> from plone.registry.tests import IMailSettings
>>> registry.register_interface(IMailSettings)

Deleting a record should fire an IRecordRemovedEvent:

>>> del registry.records['plone.registry.tests.cms']

Changing a record should fire an IRecordModifiedEvent:

>>> registry['plone.registry.tests.age'] = 25
>>> registry.records['plone.registry.tests.age'].value = 24

Let's take a look at the events that were just fired:

>>> from plone.registry.interfaces import IRecordEvent
>>> from zope.component.eventtesting import getEvents
>>> getEvents(IRecordEvent)
[<RecordAddedEvent for plone.registry.tests.age>,
<RecordAddedEvent for plone.registry.tests.cms>,
<RecordAddedEvent for plone.registry.tests.IMailSettings.sender>,
<RecordAddedEvent for plone.registry.tests.IMailSettings.smtp_host>,
<RecordRemovedEvent for plone.registry.tests.cms>,
<RecordModifiedEvent for plone.registry.tests.age>,
<RecordModifiedEvent for plone.registry.tests.age>]

For the modified events, we can also check the value before and after the

>>> from plone.registry.interfaces import IRecordModifiedEvent
>>> [(repr(e), e.old_value, e.new_value,) for e in getEvents(IRecordModifiedEvent)]
[('<RecordModifiedEvent for plone.registry.tests.age>', 18, 25),
('<RecordModifiedEvent for plone.registry.tests.age>', 25, 24)]

IObjectEvent-style redispatchers

There is a special event handler which takes care of re-dispatching registry
events based on the schema interface prescribed by the record.

Let's re-set the event testing framework and register the re-dispatching event
subscriber. Normally, this would happen automatically by including this
package's ZCML.

>>> from zope.component.eventtesting import clearEvents
>>> from zope.component import provideHandler
>>> from import redispatch_interface_aware_record_events
>>> clearEvents()
>>> provideHandler(redispatch_interface_aware_record_events)

We'll then register a schema interface.

>>> from plone.registry.tests import IMailSettings
>>> registry.register_interface(IMailSettings)

We could now register an event handler to print any record event occurring on
an IMailSettings record. More specialised event handlers for e.g.
IRecordModifiedEvent or IRecordRemovedEvent are of course also possible.
Note that it is not possible to re-dispatch IRecordAddedEvents, so these are
never caught.

>>> from zope.component import adapter
>>> @adapter(IMailSettings, IRecordEvent)
... def print_mail_settings_events(proxy, event):
... print "Got", event, "for", proxy
>>> provideHandler(print_mail_settings_events)

Let's now modify one of the records for this interface. The event handler
should react immediately.

>>> registry['plone.registry.tests.IMailSettings.sender'] = u"Some sender"
Got <RecordModifiedEvent for plone.registry.tests.IMailSettings.sender> for <RecordsProxy for plone.registry.tests.IMailSettings>

Let's also modify a non-interface-aware record, for comparison's sake. Here,
there is nothing printed.

>>> registry['plone.registry.tests.age'] = 3

We can try a record-removed event as well:

>>> del registry.records['plone.registry.tests.IMailSettings.sender']
Got <RecordRemovedEvent for plone.registry.tests.IMailSettings.sender> for <RecordsProxy for plone.registry.tests.IMailSettings>

The basic events that have been dispatched are:

>>> getEvents(IRecordEvent)
[<RecordAddedEvent for plone.registry.tests.IMailSettings.sender>,
<RecordAddedEvent for plone.registry.tests.IMailSettings.smtp_host>,
<RecordModifiedEvent for plone.registry.tests.IMailSettings.sender>,
<RecordModifiedEvent for plone.registry.tests.age>,
<RecordRemovedEvent for plone.registry.tests.IMailSettings.sender>]

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