Marshmallow

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Features:

- *Backwards-incompatible*: Rework ``ValidationError`` API.
It now expects a single field name, and error structures are merged
in the final ``ValidationError`` raised when validation completes.
This allows schema-level validators to raise errors for individual
fields (:issue:`441`). Thanks :user:`maximkulkin` for
writing the original ``merge_errors`` implementation in :pr:`442` and thanks
:user:`lafrech` for completing the implementation in :pr:`1026`.

Bug fixes:

- Fix ``TypeError`` when serializing ``None`` with ``Pluck`` (:pr:`1049`).
Thanks :user:`toffan` for the catch and patch.

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Bug fixes:

- Includes bug fixes from 2.16.2 and 2.16.3.

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Features:

- Support partial loading of nested fields (:pr:`438`). Thanks
:user:`arbor-dwatson` for the PR. *Note*: Subclasses of ``fields.Nested``
now take an additional ``partial`` parameter in the ``_deserialize``
method.

Bug fixes:

- Restore ``Schema.TYPE_MAPPING``, which was removed in 3.0.0b17 (:issue:`1012`).

Other changes:

- *Backwards-incompatible*: ``_serialize`` and ``_deserialize`` methods of
all ``fields.Field`` subclasses must accept ``**kwargs`` (:pr:`1007`).

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Bug fixes:

- Fix ``Date`` deserialization when using custom format (:issue:`1001`). Thanks
:user:`Ondkloss` for reporting.

Deprecations/Removals:

- ``prefix`` parameter or ``Schema`` class is removed (:issue:`991`). The same
can be achieved using a ``post_dump`` method.

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Features:

- Add ``format`` option to ``Date`` field (:pr:`869`).
- *Backwards-incompatible*: Rename ``DateTime``'s ``dateformat`` Meta option
to ``datetimeformat``. ``dateformat`` now applies to ``Date`` (:pr:`869`).
Thanks :user:`knagra` for implementing these changes.
- Enforce ISO 8601 when deserializing date and time (:issue:`899`).
Thanks :user:`dushr` for the report and the work on the PR.
- *Backwards-incompatible*: Raise ``ValueError`` on ``Schema`` instantiation in
case of ``attribute`` or ``data_key`` collision (:pr:`992`).

Bug fixes:

- Fix inconsistencies in field inference by refactoring the inference feature
into a dedicated field (:issue:`809`). Thanks :user:`taion` for the PR.
- When ``unknown`` is not passed to ``Nested``, default to nested ``Schema``
``unknown`` meta option rather than ``RAISE`` (:pr:`963`).
Thanks :user:`vgavro` for the PR.
- Fix loading behavior of ``fields.Pluck`` (:pr:`990`).
- Includes bug fix from 2.16.0.

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Bug fixes:

- Fix ``root`` attribute for nested container fields
on inheriting schemas (:issue:`956`). Thanks :user:`bmcbu`
for reporting.