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Sources
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Concepts
--------

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
ITitleTokenizedTerm interface contains a triple of (value, token, term).

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
    >>> class MySource(object):
    ...     interface.implements(ISource)
    ...     divisor = 2
    ...     def __contains__(self, value):
    ...         return bool(value % self.divisor)
    >>> my_source = MySource()
    >>> 1 in my_source
    True
    >>> 2 in my_source
    False

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

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.

    >>> def my_binder(context):
    ...     print "Binder was called."
    ...     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())
    Binder was called.
    >>> bound.vocabulary
    <...MySource...>
    >>> bound.vocabulary.divisor
    3

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)

    >>> bound = choice.bind(Context())
    Binder was 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.
