Marshmallow

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Bug fixes:

- Skip field validators for fields that aren't included in ``only`` (:issue:`320`). Thanks :user:`carlos-alberto` for reporting and :user:`eprikazc` for the PR.

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Features:

- Add support for partial deserialization with the ``partial`` argument to ``Schema`` and ``Schema.load`` (:issue:`290`). Thanks :user:`taion`.

Deprecation/Removals:

- ``Query`` and ``QuerySelect`` fields are removed.
- Passing of strings to ``required`` and ``allow_none`` is removed. Pass the ``error_messages`` argument instead.

Support:

- Add example of Schema inheritance in docs (:issue:`225`). Thanks :user:`martinstein` for the suggestion and :user:`juanrossi` for the PR.
- Add "Customizing Error Messages" section to custom fields docs.

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Bug fixes:

- Fix serialization of collections for which ``iter`` will modify position, e.g. Pymongo cursors (:issue:`303`). Thanks :user:`Mise` for the catch and patch.

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Bug fixes:

- Fix passing data to schema validator when using ``validates_schema(many=True)`` (:issue:`297`). Thanks :user:`d-sutherland` for reporting.
- Fix usage of ``validates`` with a nested field when ``many=True`` (:issue:`298`). Thanks :user:`nelfin` for the catch and patch.

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Bug fixes:

- ``Constant`` field deserializes to its value regardless of whether its field name is present in input data (:issue:`291`). Thanks :user:`fayazkhan` for reporting.

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Features:

- Add ``Dict`` field for arbitrary mapping data (:issue:`251`). Thanks :user:`dwieeb` for adding this and :user:`Dowwie` for the suggestion.
- Add ``Field.root`` property, which references the field's Schema.

Deprecation/Removals:

- The ``extra`` param of ``Schema`` is deprecated. Add extra data in a ``post_load`` method instead.
- ``UnmarshallingError`` and ``MarshallingError`` are removed.

Bug fixes:

- Fix storing multiple schema-level validation errors (:issue:`287`). Thanks :user:`evgeny-sureev` for the patch.
- If ``missing=None`` on a field, ``allow_none`` will be set to ``True``.

Other changes:

- A ``List's`` inner field will have the list field set as its parent. Use ``root`` to access the ``Schema``.