Sources are designed with three concepts:

  • The source itself - an iterable

    This can return any kind of object it wants. It doesn’t have to care for browser representation, encoding, …

  • A way to map a value from the iterable to something that can be used for form values - this is called a token. A token is commonly a (unique) 7bit representation of the value.

  • A way to map a value to something that can be displayed to the user - this is called a title

The last two elements are dispatched using a so called term. The ITitledTokenizedTerm interface contains a triple of (value, token, title).

Additionally there are some lookup functions to perform the mapping between values and terms and tokens and terms.

Sources that require context use a special factory: a context source binder that is called with the context and instanciates the source when it is actually used.

Sources in Fields

A choice field can be constructed with a source or source name. When a source is used, it will be used as the source for valid values.

Create a source for all odd numbers.

>>> from zope import interface
>>> from zope.schema.interfaces import ISource, IContextSourceBinder
>>> @interface.implementer(ISource)
... class MySource(object):
...     divisor = 2
...     def __contains__(self, value):
...         return bool(value % self.divisor)
>>> my_source = MySource()
>>> 1 in my_source
>>> 2 in my_source

>>> from zope.schema import Choice
>>> choice = Choice(__name__='number', source=my_source)
>>> bound = choice.bind(object())
>>> bound.vocabulary

If a IContextSourceBinder is passed as the source argument to Choice, it’s bind method will be called with the context as its only argument. The result must implement ISource and will be used as the source.

>>> _my_binder_called = []
>>> def my_binder(context):
...     _my_binder_called.append(context)
...     source = MySource()
...     source.divisor = context.divisor
...     return source
>>> interface.directlyProvides(my_binder, IContextSourceBinder)

>>> class Context(object):
...     divisor = 3

>>> choice = Choice(__name__='number', source=my_binder)
>>> bound = choice.bind(Context())
>>> len(_my_binder_called)
>>> bound.vocabulary
>>> bound.vocabulary.divisor

When using IContextSourceBinder together with default value, it’s impossible to validate it on field initialization. Let’s check if initalization doesn’t fail in that case.

>>> choice = Choice(__name__='number', source=my_binder, default=2)

>>> del _my_binder_called[:]
>>> bound = choice.bind(Context())
>>> len(_my_binder_called)

>>> bound.validate(bound.default)
>>> bound.validate(3)
Traceback (most recent call last):
ConstraintNotSatisfied: 3

It’s developer’s responsibility to provide a default value that fits the constraints when using context-based sources.