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This document highlights unusual and subtle aspects of various fields and field classes, and is not intended to be a general introduction to schema fields. Please see README.txt for a more general introduction.

While many field types, such as Int, TextLine, Text, and Bool are relatively straightforward, a few have some subtlety. We will explore the general class of collections and discuss how to create a custom creation field; discuss Choice fields, vocabularies, and their use with collections; and close with a look at the standard zope.app approach to using these fields to find views ("widgets").


Normal fields typically describe the API of the attribute -- does it behave as a Python Int, or a Float, or a Bool -- and various constraints to the model, such as a maximum or minimum value. Collection fields have additional requirements because they contain other types, which may also be described and constrained.

For instance, imagine a list that contains non-negative floats and enforces uniqueness. In a schema, this might be written as follows:

>>> from zope.interface import Interface
>>> from zope.schema import List, Float
>>> class IInventoryItem(Interface):
...     pricePoints = List(
...         title=u"Price Points",
...         unique=True,
...         value_type=Float(title=u"Price", min=0.0)
...     )

This indicates several things.

This declaration creates a field that implements a number of interfaces, among them these:

>>> from zope.schema.interfaces import IList, ISequence, ICollection
>>> IList.providedBy(IInventoryItem['pricePoints'])
>>> ISequence.providedBy(IInventoryItem['pricePoints'])
>>> ICollection.providedBy(IInventoryItem['pricePoints'])

Creating a custom collection field

Ideally, custom collection fields have interfaces that inherit appropriately from either zope.schema.interfaces.ISequence or zope.schema.interfaces.IUnorderedCollection. Most collection fields should be able to subclass zope.schema._field.AbstractCollection to get the necessary behavior. Notice the behavior of the Set field in zope.schema._field: this would also be necessary to implement a Bag.

Choices and Vocabularies

Choice fields are the schema way of spelling enumerated fields and more. By providing a dynamically generated vocabulary, the choices available to a choice field can be contextually calculated.

Simple choices do not have to explicitly use vocabularies:

>>> from zope.schema import Choice
>>> f = Choice((640, 1028, 1600))
>>> f.validate(640)
>>> f.validate(960)
Traceback (most recent call last):
ConstraintNotSatisfied: 960
>>> f.validate('bing')
Traceback (most recent call last):
ConstraintNotSatisfied: bing

More complex choices will want to use registered vocabularies. Vocabularies have a simple interface, as defined in zope.schema.interfaces.IBaseVocabulary. A vocabulary must minimally be able to determine whether it contains a value, to create a term object for a value, and to return a query interface (or None) to find items in itself. Term objects are an abstraction that wraps a vocabulary value.

The Zope application server typically needs a fuller interface that provides "tokens" on its terms: ASCII values that have a one-to-one relationship to the values when the vocabulary is asked to "getTermByToken". If a vocabulary is small, it can also support the IIterableVocabulary interface.

If a vocabulary has been registered, then the choice merely needs to pass the vocabulary identifier to the "vocabulary" argument of the choice during instantiation.

A start to a vocabulary implementation that may do all you need for many simple tasks may be found in zope.schema.vocabulary.SimpleVocabulary. Because registered vocabularies are simply callables passed a context, many registered vocabularies can simply be functions that rely on SimpleVocabulary:

>>> from zope.schema.vocabulary import SimpleVocabulary
>>> def myDynamicVocabulary(context):
...     v = dynamic_context_calculation_that_returns_an_iterable(context)
...     return SimpleVocabulary.fromValues(v)

The vocabulary interface is simple enough that writing a custom vocabulary is not too difficult itself.

Choices and Collections

Choices are a field type and can be used as a value_type for collections. Just as a collection of an "Int" value_type constrains members to integers, so a choice-based value type constrains members to choices within the Choice's vocabulary. Typically in the Zope application server widgets are found not only for the collection and the choice field but also for the vocabulary on which the choice is based.

Using Choice and Collection Fields within a Widget Framework

While fields support several use cases, including code documentation and data description and even casting, a significant use case influencing their design is to support form generation -- generating widgets for a field. Choice and collection fields are expected to be used within widget frameworks. The zope.app approach typically (but configurably) uses multiple dispatches to find widgets on the basis of various aspects of the fields.

Widgets for all fields are found by looking up a browser view of the field providing an input or display widget view. Typically there is only a single "widget" registered for Choice fields. When it is looked up, it performs another dispatch -- another lookup -- for a widget registered for both the field and the vocabulary. This widget typically has enough information to render without a third dispatch.

Collection fields may fire several dispatches. The first is the usual lookup by field. A single "widget" should be registered for ICollection, which does a second lookup by field and value_type constraint, if any, or, theoretically, if value_type is None, renders some absolutely generic collection widget that allows input of any value imaginable: a check-in of such a widget would be unexpected. This second lookup may find a widget that knows how to render, and stop. However, the value_type may be a choice, which will usually fire a third dispatch: a search for a browser widget for the collection field, the value_type field, and the vocabulary. Further lookups may even be configured on the basis of uniqueness and other constraints.

This level of indirection may be unnecessary for some applications, and can be disabled with simple ZCML changes within zope.app.