Default allow_null=True for Nested fields

[close #132]

HISTORY.rst

@@ -25,6 +25,7 @@ Deprecation/Removals:
 Other changes:
 
 - ``Marshaller``, ``Unmarshaller``, ``null``, and ``missing`` were moved to ``marshmallow.marshalling``. These should be considered private API (:issue:`129`).
+- Make ``allow_null=True`` the default for ``Nested`` fields. This will make ``None`` serialize to ``None`` rather than a dictionary with empty values (:issue:`132`). Thanks :user:`nickrellack` for the suggestion.
 
 1.2.2 (unreleased)
 ++++++++++++++++++

marshmallow/fields.py

@@ -316,13 +316,13 @@ class Nested(Field):
         will be marshalled. If a field name (string) is given, only a single
         value will be returned as output instead of a dictionary.
         This parameter takes precedence over ``exclude``.
-    :param bool allow_null: Whether to return None instead of a dictionary
-        with null keys, if a nested dictionary has all-null keys
+    :param bool allow_null: Whether to return `None` instead of a dictionary
+        with null keys, if a nested dictionary has all-null keys.
     :param bool many: Whether the field is a collection of objects.
     :param kwargs: The same keyword arguments that :class:`Field` receives.
     """
 
-    def __init__(self, nested, default=null, exclude=tuple(), only=None, allow_null=False,
+    def __init__(self, nested, default=null, exclude=tuple(), only=None, allow_null=True,
                 many=False, **kwargs):
         self.nested = nested
         self.allow_null = allow_null
@@ -371,6 +371,9 @@ def schema(self):
         return self.__schema
 
     def _serialize(self, nested_obj, attr, obj):
+        # Load up the schema first. This allows a RegistryError to be raised
+        # if an invalid schema name was passed
+        schema = self.schema
         if nested_obj is None:
             if self.many:
                 return []

tests/test_registry.py

@@ -100,12 +100,12 @@ def test_multiple_classes_with_all():
 def test_can_use_full_module_path_to_class():
     from .foo_serializer import FooSerializer as FooSerializer1  # noqa
     # Using full paths is ok
-    field = fields.Nested('tests.foo_serializer.FooSerializer')
+    field = fields.Nested('tests.foo_serializer.FooSerializer', allow_null=False)
 
     # Note: The arguments here don't matter. What matters is that no
     # error is raised
     assert field.serialize('bar', {'foo': {'_id': 42}})
 
-    field2 = fields.Nested('tests.test_registry.FooSerializer')
+    field2 = fields.Nested('tests.test_registry.FooSerializer', allow_null=False)
 
     assert field2.serialize('bar', {'foo': {'_id': 42}})

tests/test_schema.py

@@ -1420,7 +1420,7 @@ class MySchema2(Schema):
 
         s = MySchema()
         result = s.dump({'foo': None})
-        assert result.data['foo'] == {'bar': None}
+        assert result.data['foo'] is None
 
         s2 = MySchema2()
         result2 = s2.dump({'foo': None})