Features
~~~~~~~~

- Calling ``bind`` on a schema node e.g. ``cloned_node = somenode.bind(a=1,
  b=2)`` on a schema node now results in the cloned node having a
  ``bindings`` attribute of the value ``{'a':1, 'b':2}``.

- It is no longer necessary to pass a ``typ`` argument to a SchemaNode
  constructor if the node class has a ``__schema_type__`` callable as a class
  attribute which, when called with no arguments, returns a schema type.
  This callable will be called to obtain the schema type if a ``typ`` is not
  supplied to the constructor.  The default ``SchemaNode`` object's
  ``__schema_type__`` callable raises a ``NotImplementedError`` when it is
  called.

- SchemaNode now has a ``raise_invalid`` method which accepts a message and
  raises a colander.Invalid exception using ``self`` as the node and the
  message as its message.

- It is now possible and advisable to subclass ``SchemaNode`` in order to
  create a bundle of default node behavior.  The subclass can define the
  following methods and attributes: ``preparer``, ``validator``, ``default``,
  ``missing``, ``name``, ``title``, ``description``, ``widget``, and
  ``after_bind``.  For example, the older, more imperative style that
  looked like this still works::

     from colander import SchemaNode

     ranged_int = colander.SchemaNode(
         validator=colander.Range(0, 10),
         default = 10,
         title='Ranged Int'
         )

  But you can alternately now do something like::

     from colander import SchemaNode

     class RangedIntSchemaNode(SchemaNode):
         validator = colander.range(0, 10)
         default = 10
         title = 'Ranged Int'

     ranged_int = RangedInt()

   Values that are expected to be callables can be methods of the schemanode
   subclass instead of plain attributes::

     from colander import SchemaNode

     class RangedIntSchemaNode(SchemaNode):
         default = 10
         title = 'Ranged Int'

         def validator(self, node, cstruct):
            if not 0 < cstruct < 10:
                raise colander.Invalid(node, 'Must be between 0 and 10')

     ranged_int = RangedInt()

   When implementing a method value that expects ``node``, ``node`` must be
   provided in the call signature, even though ``node`` will almost always be
   the same as ``self``.  This is because Colander simply treats the method
   as another kind of callable, be it a method, or a function, or an instance
   that has a ``__call__`` method.  It doesn't care that it happens to be a
   method of ``self``, and it needs to support callables that are not
   methods, so it sends ``node`` in regardless.

   Normal inheritance rules apply to class attributes and methods defined in
   a schemanode subclass.  If your schemanode subclass inherits from another
   schemanode class, your schemanode subclass' methods and class attributes
   will override the superclass' methods and class attributes.

   Method values that need to be deferred for binding cannot currently be
   implemented as ``colander.deferred`` callables.  For example this will
   *not* work::

     from colander import SchemaNode

     class RangedIntSchemaNode(SchemaNode):
         default = 10
         title = 'Ranged Int'

         @colander.deferred
         def validator(self, node, kw):
            request = kw['request']
            def avalidator(node, cstruct):
                if not 0 < cstruct < 10:
                    if request.user != 'admin':
                        raise colander.Invalid(node, 'Must be between 0 and 10')
            return avalidator

     ranged_int = RangedInt()
     bound_ranged_int = ranged_int.bind(request=request)

   This will result in::

        TypeError: avalidator() takes exactly 3 arguments (2 given)

   Instead of trying to defer methods via a decorator, you can instead use
   the ``bindings`` attribute of ``self`` to obtain access to the bind
   parameters within values that are methody::

     from colander import SchemaNode

     class RangedIntSchemaNode(SchemaNode):
         default = 10
         title = 'Ranged Int'

         def validator(self, node, cstruct):
            request = self.bindings['request']
            if not 0 < cstruct < 10:
                if request.user != 'admin':
                    raise colander.Invalid(node, 'Must be between 0 and 10')

     ranged_int = RangedInt()
     bound_range_int = ranged_int.bind(request=request)

   You can use ``after_bind`` to set attributes of the schemanode that rely
   on binding variables, such as ``missing`` and ``default``::

     from colander import SchemaNode

     class RangedIntSchemaNode(SchemaNode):
         default = 10
         title = 'Ranged Int'

         def validator(self, node, cstruct):
            request = self.bindings['request']
            if not 0 < cstruct < 10:
                if request.user != 'admin':
                    raise colander.Invalid(node, 'Must be between 0 and 10')

         def after_bind(self, node, kw):
             self.request = kw['request']
             self.default = self.request.user.id

   Non-method values can still be implemented as ``colander.deferred``
   however::

     from colander import SchemaNode

     def _missing(node, kw):
         request = kw['request']
         if request.user.name == 'admin':
             return 10
          return 20

     class RangedIntSchemaNode(SchemaNode):
         default = 10
         title = 'Ranged Int'
         missing = colander.deferred(_missing)

     ranged_int = RangedInt()

   You can override the default values of a schemanode subclass in its
   constructor::

     from colander import SchemaNode

     class RangedIntSchemaNode(SchemaNode):
         default = 10
         title = 'Ranged Int'
         validator = colander.Range(0, 10)

     ranged_int = RangedInt(validator=colander.Range(0, 20))

   In the above example, the validation will be done on 0-20, not 0-10.

   If your schema node names conflict with schema value attribute names, you
   can work around it with the ``name`` argument to the schema node::

     from colander import SchemaNode, Schema

     class TitleNode(SchemaNode):
         validator = colander.range(0, 10)
         default = 10

     class SomeSchema(Schema):
         title = 'Some Schema'
         thisnamewontmatter = TitleNode(name='title')

Backwards Incompatibilities
~~~~~~~~~~~~~~~~~~~~~~~~~~~

- Passing non-SchemaNode derivative instances as ``*children`` into a
  SchemaNode constructor is no longer supported.  Symptom: ``AttributeError:
  name`` when constructing a SchemaNode.