Marshmallow

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Adds new features, speed improvements, better error handling, and updated documentation.

- Add ``skip_missing`` ``class Meta`` option.
- A field's ``default`` may be a callable.
- Allow accessor function to be configured via the ``Schema.accessor`` decorator or the ``__accessor__`` class member.
- ``URL`` and ``Email`` fields are validated upon serialization.
- ``dump`` and ``load`` can receive the ``many`` argument.
- Move a number of utility functions from fields.py to utils.py.
- More useful ``repr`` for ``Field`` classes.
- If a field's default is ``fields.missing`` and its serialized value is ``None``, it will not be included in the final serialized result.
- Schema.dumps no longer coerces its result to a binary string on Python 3.
- *Backwards-incompatible*: Schema output is no longer an ``OrderedDict`` by default. If you want ordered field output, you must explicitly set the ``ordered`` option to ``True``.
- *Backwards-incompatible*: ``error`` parameter of the ``Field`` constructor is deprecated. Raise a ``ValidationError`` instead.
- Expanded test coverage.
- Updated docs.

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Major reworking and simplification of the public API, centered around support for deserialization, improved validation, and a less stateful ``Schema`` class.

* Rename ``Serializer`` to ``Schema``.
* Support for deserialization.
* Use the ``Schema.dump`` and ``Schema.load`` methods for serializing and deserializing, respectively.
* *Backwards-incompatible*: Remove ``Serializer.json`` and ``Serializer.to_json``. Use ``Schema.dumps`` instead.
* Reworked fields interface.
* *Backwards-incompatible*: ``Field`` classes implement ``_serialize`` and ``_deserialize`` methods. ``serialize`` and ``deserialize`` comprise the public API for a ``Field``. ``Field.format`` and ``Field.output`` have been removed.
* Add ``exceptions.ForcedError`` which allows errors to be raised during serialization (instead of storing errors in the ``errors`` dict).
* *Backwards-incompatible*: ``DateTime`` field serializes to ISO8601 format by default (instead of RFC822).
* *Backwards-incompatible*: Remove ``Serializer.factory`` method. It is no longer necessary with the ``dump`` method.
* *Backwards-incompatible*: Allow nesting a serializer within itself recursively. Use ``exclude`` or ``only`` to prevent infinite recursion.
* *Backwards-incompatible*: Multiple errors can be stored for a single field. The errors dictionary returned by ``load`` and ``dump`` have lists of error messages keyed by field name.
* Remove ``validated`` decorator. Validation occurs within ``Field`` methods.
* ``Function`` field raises a ``ValueError`` if an uncallable object is passed to its constructor.
* ``Nested`` fields inherit context from their parent.
* Add ``Schema.preprocessor`` and ``Schema.validator`` decorators for registering preprocessing and schema-level validation functions respectively.
* Custom error messages can be specified by raising a ``ValidationError`` within a validation function.
* Extra keyword arguments passed to a Field are stored as metadata.
* Fix ordering of field output.
* Fix behavior of the ``required`` parameter on ``Nested`` fields.
* Fix serializing keyed tuple types (e.g. ``namedtuple``) with ``class Meta`` options.
* Fix default value for ``Fixed`` and ``Price`` fields.
* Fix serialization of binary strings.
* ``Schemas`` can inherit fields from non-``Schema`` base classes (e.g. mixins). Also, fields are inherited according to the MRO (rather than recursing over base classes). Thanks :user:`jmcarp`.
* Add ``Str``, ``Bool``, and ``Int`` field class aliases.

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* Add ``Serializer.error_handler`` decorator that registers a custom error handler.
* Add ``Serializer.data_handler`` decorator that registers data post-processing callbacks.
* *Backwards-incompatible*: ``process_data`` method is deprecated. Use the ``data_handler`` decorator instead.
* Fix bug that raised error when passing ``extra`` data together with ``many=True``. Thanks :user:`buttsicles` for reporting.
* If ``required=True`` validation is violated for a given ``Field``, it will raise an error message that is different from the message specified by the ``error`` argument. Thanks :user:`asteinlein`.
* More generic error message raised when required field is missing.
* ``validated`` decorator should only wrap a ``Field`` class's ``output`` method.

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* Fix bug in serializing keyed tuple types, e.g. ``namedtuple`` and ``KeyedTuple``.
* Nested field can load a serializer by its class name as a string. This makes it easier to implement 2-way nesting.
* Make ``Serializer.data`` override-able.

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* Add ``Serializer.factory`` for creating a factory function that returns a Serializer instance.
* ``MarshallingError`` stores its underlying exception as an instance variable. This is useful for inspecting errors.
* ``fields.Select`` is aliased to ``fields.Enum``.
* Add ``fields.__all__`` and ``marshmallow.__all__`` so that the modules can be more easily extended.
* Expose ``Serializer.OPTIONS_CLASS`` as a class variable so that options defaults can be overridden.
* Add ``Serializer.process_data`` hook that allows subclasses to manipulate the final output data.

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* Add ``json_module`` class Meta option.
* Add ``required`` option to fields . Thanks :user:`DeaconDesperado`.
* Tested on Python 3.4 and PyPy.