plone.app.registry 1.0.1

Creating records

Once you have decided that you need a particular record, you need to answer two questions:

1. What should the record be called?

2. What type of data should it hold?

Let’s say you wanted to create a record call my.package.timeout, holding an integer. Integers are described by the field type plone.registry.field.Int. Almost all the standard fields you would find in zope.schema have an equivalent field in plone.registry.field. The main exception is Object, which is unsupported. Also, Choice fields only support vocabularies given by string name, or as a list of string values. Finally, you cannot use the constraint property to set a validator function, although other validation (such as min/max values) will work.

To install such a record, you could add a registry.xml step to the GenericSetup profile of your product like this:

<registry>

    <record name="my.package.timeout">
        <field type="plone.registry.field.Int">
            <title>Timeout</title>
            <min>0</min>
        </field>
        <value>100</value>
    </record>

</registry>

Let’s look at this in more detail:

• There is one record declared. The name is given in the name attribute.

• In the record, we first define the field type, by giving the full dotted name to the field class. Unless you have installed a third party package providing additional persistent fields, this will be a class in plone.registry.field mirroring a corresponding class in zope.schema.

• Inside the <field /> element, we list any required or optional attributes of the field. This uses plone.supermodel syntax. In essence, each allowed field attribute is represented by a tag (so the title attribute can be set with the <title /> tag), with the attribute value given as the tag body. If an attribute is required for a field, the corresponding tag is required here.

• We then set the value. This must obviously be a valid value for the field type.

Note that the <value /> is optional. If not given, the field will default to its missing_value until it is set. The <field /> is optional if the record has already been initialised elsewhere.

Most field attributes are simple tags like the ones shown above, with the field name used as the tag name, and a string representation of the value used as the contents of the tag. Collection fields are a little more involved, however. A collection field (like a List or Tuple) has a value_type property containing another field. Also, their values and defaults are sequences. Let’s look at an example:

<record name="my.package.animals">
    <field type="plone.registry.field.Tuple">
        <title>Animals</title>
        <description>A list of cool animals</description>
        <value_type type="plone.registry.field.TextLine" />
    </field>
    <value>
        <element>Dog</element>
        <element>Cat</element>
        <element>Elephant</element>
    </value>
</record>

Notice how the <value_type /> tag takes a type attribute just like the outer <field /> tag. Here we have shown a value type with no options, but if you need, you can put tags for additional field attributes inside the <value_type /> tag.

Also notice how the value is represented. Each element in the sequence (a tuple in this case) is given by an <element /> tag, with the element value given as the body of that tag.

Dict fields also have a <key_type /> and elements that are key/value pairs. They can be configured like so:

<record name="my.package.animalFood">
    <field type="plone.registry.field.Dict">
        <title>Food eaten by animals</title>
        <key_type type="plone.registry.field.TextLine" />
        <value_type type="plone.registry.field.TextLine" />
    </field>
    <value>
        <element key="Dog">Dog food</element>
        <element key="Cat">Cat food</element>
        <element key="Elephant">Squirrels</element>
    </value>
</record>

Field references

It is possible to define record to use another record’s field. This is often useful if you want one record to act as an optional override for another. For example:

<registry>

    <record name="my.package.timeout">
        <field type="plone.registry.field.Int">
            <title>Timeout</title>
            <min>0</min>
        </field>
        <value>100</value>
    </record>

    <record name="my.package.timeout.slowconnection">
        <field ref="my.package.timeout" />
        <value>300</value>
    </record>

</registry>

In this example, we have defined the my.package.timeout record with an integer field. We then have a separate record, with a separate value, called my.package.timeout.slowconnection, which uses the same field (with the same type, validation, title, description, etc). This avoids having to explicitly re-define a complete field.

Note: The field in this case is actually a FieldRef object. See the plone.registry documentation for details.

Setting values

Once a record has been defined, its value can be set or updated using GenericSetup like so:

<record name="my.package.animalFood">
    <value purge="false">
        <element key="Squirrel">Nuts</element>
        <element key="Piranha">Other piranha</element>
    </value>
</record>

This is often useful if you have a record defined in one package that is appended to or customised in another package.

In the example above, we used the purge attribute. When setting the value of a multi-valued field such as a tuple, list, set or dictionary, setting this attribute to false will cause the values listed to be added to the existing collection, rather than overriding the collection entirely, as would happen if the purge attribute was set to true or omitted.

Deleting records

To delete a record, use the delete attribute:

<record name="my.package.animalFood" delete="true" />

If the record does not exist, a warning will be logged, but processing will continue.

Creating records based on an interface

In the examples above, we created individual records directly in the registry. Sometimes, however, it is easier to work with traditional schema interfaces that group together several related fields. As we will see below, plone.registry and plone.app.registry provide certain additional functionality for groups of records created from an interface.

For example, we could have an interface like this:

from zope.interface import Interface
from zope import schema

class IZooSettings(Interface):

    entryPrice = schema.Decimal(title=u"Admission charge")
    messageOfTheDay = schema.TextLine(title=u"A banner message", default=u"Welcome!")

Notice how we are using standard zope.schema fields. These will be converted to persistent fields (by adapting them to IPersistentField from plone.registry) when the registry is populated. If that is not possible, an error will occur on import.

To register these records, we simply add the following to registry.xml:

<records interface="my.package.interfaces.IZooSettings" />

This will create one record for each field. The record names are the full dotted names to the fields, so in this case they would be my.package.interfaces.IZooSettings.entryPrice and my.package.interfaces.IZooSettings.messageOfTheDay.

If you just want to use the interface as a template you can supply a prefix attribute:

<records interface=”my.package.interfaces.IZooSettings” prefix=”my.zoo” />

which will generate fields named my.zoo.entryPrice and my.zoo.messageOfTheDay.

In order to set the values of the fields created by a <records /> directive you must provide value entries with keys corresponding to the fields on the interface, as follows:

<records interface="my.package.interfaces.IZooSettings" prefix="my.zoo">
    <value key="entryPrice">40</value>
    <value key="messageOfTheDay">We've got lions and tigers!</value>
</records>

Values can be set as above using the full record name. However, we can also explicitly state that we are setting a record bound to an interface, like so:

<record interface="my.package.interfaces.IZooSettings" field="entryPrice">
    <value>10.0</value>
</record>

This is equivalent to:

<record interface="my.package.interfaces.IZooSettings.entryPrice">
    <value>10.0</value>
</record>

You can also use the interface/field syntax to register a new record from an individual field.

Finally, if the interface contains fields that cannot or should be set, they may be omitted:

<records interface="my.package.interfaces.IZooSettings">
    <omit>someField</omit>
</records>

The <omit /> tag can be repeated to exclude multiple fields.

Accessing the registry

To get or set the value of a record, we must first look up the registry itself. The registry is registered as a local utility, so we can look it up with:

from zope.component import getUtility
from plone.registry.interfaces import IRegistry

registry = getUtility(IRegistry)

Values can now get read or set using simple dictionary syntax:

timeout = registry['my.package.timeout']

We can also use get() to get the value conditionally, and an in check to test whether the registry contains a particular record.

The returned value will by of a type consistent with the field for the record with the given name. It can be set in the same manner:

registry['my.package.timeout'] = 120

If you need to access the underlying record, use the records attribute:

timeoutRecord = registry.records['my.package.timeout']

The record returned conforms to plone.registry.interfaces.IRecord and has two main attributes: value is the current record value, and field is the persistent field instance. If the record was created from an interface, it will also provide IInterfaceAwareRecord and have three additional attributes: interfaceName, the string name of the interface; interface, the interface instance itself, and fieldName, the name of the field in the interface from which this record was created.

In unit tests, it may be useful to create a new record programmatically. You can do that like so:

from plone.registry.record import Record
from plone.registry import field

registry['my.record'] = Record(field.TextLine(title=u"A record"), u"Test")

The constructor takes a persistent field and the initial value as parameters.

To register records for an interface programmatically, we can do:

registry.registerInterface(IZooSettings)

You can omit fields by passing an omit parameter giving a sequence of omitted field names.

See plone.registry for more details about how to introspect and manipulate the registry records programmatically.

Using the records proxy

Above, we used dictionary syntax to access individual records and values. This will always work, but for so-called interface-aware records - those which were created from an interface e.g. using the <records /> syntax - we have another option: the records proxy. This allows us to look up all the records that belong to a particular interface at the same time, returning an object that provides the given interface and can be manipulated like an object, that is still connected to the underlying registry.

To look up a records proxy for our IZooSettings interface, we can do:

zooSettings = registry.forInterface(IZooSettings)

The zooSettings object now provides IZooSettings. Values may be read and set using attribute notation:

zooSettings.messageOfTheDay = u"New message"
currentEntryPrice = zooSettings.entryPrice

When setting a value, it is immediately validated and written to the registry. A validation error exception may be raised if the value is not permitted by the field for the corresponding record.

When fetching the records proxy, plone.registry will by default verify that records exists for each field in the interface, and will raise an error if this is not the case. To disable this check, you can do:

zooSettings = registry.forInterface(IZooSettings, check=False)

This is sometimes useful in cases where it is not certain that the registry has been initialised. You can also omit checking for individual fields, by passing an omit parameter giving a tuple of field names.

Registry events

The registry emits events when it is modified:

• plone.registry.interfaces.IRecordAddedEvent is fired when a record has been added to the registry.

• plone.registry.interfaces.IRecordRemovedEvent is fired when a record has been removed from the registry.

• plone.registry.interfaces.IRecordModifiedEvent is fired when a record’s value is modified.

You can register subscribers for these to catch any changes to the registry. In addition, you can register an event handler that only listens to changes pertaining to records associated with specific interfaces. For example:

from zope.component import adapter
from plone.registry.interfaces import IRecordModifiedEvent

from logging import getLogger
log = getLogger('my.package)

@adapter(IZooSettings, IRecordModifiedEvent)
def detectPriceChange(settings, event):
    if record.fieldName == 'entryPrice':
        log.warning("Someone change the price from %d to %d" % (event.oldValue, event.newValue,))

See plone.registry for details about these event types.