Add changelog and docs for propagate_unknown

The changelog entry, including credit to prior related work, covers
the closed issues and describes how `propagate_unknown` is expected to
behave.

Inline documentation attempts to strike a balance between clarify and
brevity.

closes #1428, marshmallow-code#1429, #1490, marshmallow-code#1575

CHANGELOG.rst

@@ -1,6 +1,34 @@
 Changelog
 ---------
 
+3.8.0 (Unreleased)
+******************
+
+Features:
+
+- The behavior of the ``unknown`` option can be further customized with a
+  second option, ``propagate_unknown``. When set to ``True``, any nested
+  schemas will receive the same ``unknown`` value as their parent if they did
+  not set an ``unknown`` value explicitly.
+  ``propagate_unknown`` itself propagates across any number of layers of
+  nesting and cannot be disabled by a child schema if it enabled in its parent.
+  (:issue:`1490`, :issue:`1428`)
+  Thanks :user:`lmignon` and :user:`mahenzon`.
+
+.. note::
+
+  In order to retain flexibility, when a schema is being loaded with
+  ``propagate_unknown=True``, you can still set ``unknown`` explicitly on child
+  schemas. However, be aware that such a value will be propagated to any of its
+  descendants.
+
+  For example, a schema which specifies ``unknown=EXCLUDE`` will set
+  ``EXCLUDE`` in all of its children. If one of the children specifies
+  ``unknown=IGNORE``, then ``IGNORE`` will be passed to the relevant
+  grandchildren.
+  Other children of the original schema could specify ``unknown=RAISE``, and
+  this would apply to them equally well.
+
 3.7.0 (2020-07-08)
 ******************
 

src/marshmallow/fields.py

@@ -459,6 +459,10 @@ class ParentSchema(Schema):
     :param many: Whether the field is a collection of objects.
     :param unknown: Whether to exclude, include, or raise an error for unknown
         fields in the data. Use `EXCLUDE`, `INCLUDE` or `RAISE`.
+    :param propagate_unknown: If ``True``, the value for ``unknown`` will be
+        applied to any ``Nested`` fields within the nested schema. Propagates down and allows
+        ``Nested`` fields to apply their own ``unknown`` value. Note that this
+        only has an effect when there are multiple layers of nesting
     :param kwargs: The same keyword arguments that :class:`Field` receives.
     """
 

src/marshmallow/schema.py

@@ -287,6 +287,9 @@ class AlbumSchema(Schema):
         will be ignored. Use dot delimiters to specify nested fields.
     :param unknown: Whether to exclude, include, or raise an error for unknown
         fields in the data. Use `EXCLUDE`, `INCLUDE` or `RAISE`.
+    :param propagate_unknown: If ``True``, the value for ``unknown`` will be
+        applied to all ``Nested`` fields. Propagates down and allows
+        ``Nested`` fields to apply their own ``unknown`` value
 
     .. versionchanged:: 3.0.0
         `prefix` parameter removed.
@@ -364,6 +367,10 @@ class Meta:
         - ``dump_only``: Tuple or list of fields to exclude from deserialization
         - ``unknown``: Whether to exclude, include, or raise an error for unknown
             fields in the data. Use `EXCLUDE`, `INCLUDE` or `RAISE`.
+        - ``propagate_unknown`` If ``True``, the value for ``unknown`` will be
+           applied to all ``Nested`` fields. Propagates down, but if a ``Nested``
+           field sets ``unknown``, it will begin to propagate that value, not the
+           where this is set.
         - ``register``: Whether to register the `Schema` with marshmallow's internal
             class registry. Must be `True` if you intend to refer to this `Schema`
             by class name in `Nested` fields. Only set this to `False` when memory
@@ -619,6 +626,9 @@ def _deserialize(
             will be ignored. Use dot delimiters to specify nested fields.
         :param unknown: Whether to exclude, include, or raise an error for unknown
             fields in the data. Use `EXCLUDE`, `INCLUDE` or `RAISE`.
+        :param propagate_unknown: If ``True``, the value for ``unknown`` will be
+            applied to all ``Nested`` fields. Propagates down and allows
+            ``Nested`` fields to apply their own ``unknown`` value
         :param int index: Index of the item being serialized (for storing errors) if
             serializing a collection, otherwise `None`.
         :return: A dictionary of the deserialized data.
@@ -733,6 +743,9 @@ def load(
         :param unknown: Whether to exclude, include, or raise an error for unknown
             fields in the data. Use `EXCLUDE`, `INCLUDE` or `RAISE`.
             If `None`, the value for `self.unknown` is used.
+        :param propagate_unknown: If ``True``, the value for ``unknown`` will be
+            applied to all ``Nested`` fields. Propagates down and allows
+            ``Nested`` fields to apply their own ``unknown`` value
         :return: Deserialized data
 
         .. versionadded:: 1.0.0
@@ -772,6 +785,9 @@ def loads(
         :param unknown: Whether to exclude, include, or raise an error for unknown
             fields in the data. Use `EXCLUDE`, `INCLUDE` or `RAISE`.
             If `None`, the value for `self.unknown` is used.
+        :param propagate_unknown: If ``True``, the value for ``unknown`` will be
+            applied to all ``Nested`` fields. Propagates down and allows
+            ``Nested`` fields to apply their own ``unknown`` value
         :return: Deserialized data
 
         .. versionadded:: 1.0.0
@@ -864,6 +880,9 @@ def _do_load(
         :param unknown: Whether to exclude, include, or raise an error for unknown
             fields in the data. Use `EXCLUDE`, `INCLUDE` or `RAISE`.
             If `None`, the value for `self.unknown` is used.
+        :param propagate_unknown: If ``True``, the value for ``unknown`` will be
+            applied to all ``Nested`` fields. Propagates down and allows
+            ``Nested`` fields to apply their own ``unknown`` value
         :param postprocess: Whether to run post_load methods..
         :return: Deserialized data
         """