Field take default on feature

AUTHORS.rst

@@ -159,3 +159,4 @@ Contributors (chronological)
 - Stephen Rosen `@sirosen <https://github.com/sirosen>`_
 - Vladimir Mikhaylov `@vemikhaylov <https://github.com/vemikhaylov>`_
 - Stephen Eaton `@madeinoz67 <https://github.com/madeinoz67>`_
+- Dor Meiri `@dormeiri <https://github.com/dormeiri>`_

src/marshmallow/fields.py

@@ -164,6 +164,7 @@ def __init__(
         dump_only: bool = False,
         error_messages: typing.Optional[typing.Dict[str, str]] = None,
         metadata: typing.Optional[typing.Mapping[str, typing.Any]] = None,
+        take_default_on=None,
         **additional_metadata
     ) -> None:
         self.default = default
@@ -191,6 +192,7 @@ def __init__(
             raise ValueError("'missing' must not be set for required fields.")
         self.required = required
         self.missing = missing
+        self.take_default_on = tuple() if take_default_on is None else take_default_on
 
         metadata = metadata or {}
         self.metadata = {**metadata, **additional_metadata}
@@ -218,7 +220,8 @@ def __repr__(self) -> str:
             "validate={self.validate}, required={self.required}, "
             "load_only={self.load_only}, dump_only={self.dump_only}, "
             "missing={self.missing}, allow_none={self.allow_none}, "
-            "error_messages={self.error_messages})>".format(
+            "error_messages={self.error_messages}, "
+            "take_default_on={self.take_default_on})>".format(
                 ClassName=self.__class__.__name__, self=self
             )
         )
@@ -323,10 +326,10 @@ def serialize(
         """
         if self._CHECK_ATTRIBUTE:
             value = self.get_value(obj, attr, accessor=accessor)
-            if value is missing_ and hasattr(self, "default"):
+            if self._should_take_default(value) and hasattr(self, "default"):
                 default = self.default
                 value = default() if callable(default) else default
-            if value is missing_:
+            if self._should_take_default(value):
                 return value
         else:
             value = None
@@ -416,6 +419,9 @@ def _deserialize(
         """
         return value
 
+    def _should_take_default(self, value):
+        return value is missing_ or value in self.take_default_on
+
     # Properties
 
     @property

tests/test_fields.py

@@ -36,7 +36,7 @@ def test_repr(self):
             "validate=None, required=False, "
             "load_only=False, dump_only=False, "
             "missing={missing}, allow_none=False, "
-            "error_messages={error_messages})>".format(
+            "error_messages={error_messages}, take_default_on=())>".format(
                 default, missing=missing, error_messages=field.error_messages
             )
         )

tests/test_schema.py

@@ -2748,6 +2748,51 @@ class MySchema(Schema):
         assert errors["allow_none_field"][0] == "<custom>"
 
 
+class TestTakeDefaultOn:
+    class MySchema(Schema):
+        int_take_default_on_none = fields.Int(default=42, take_default_on=[None])
+        int_take_default_on_value = fields.Int(default=42, take_default_on=[0, 1])
+        str_take_default_on_none = fields.Str(default="foo", take_default_on=[None])
+        str_take_default_on_value = fields.Str(
+            default="foo", take_default_on=["", "bar"]
+        )
+
+    @pytest.fixture()
+    def schema(self):
+        return self.MySchema()
+
+    @pytest.fixture()
+    def default_values(self):
+        return dict(
+            int_take_default_on_none=42,
+            int_take_default_on_value=42,
+            str_take_default_on_none="foo",
+            str_take_default_on_value="foo",
+        )
+
+    def test_default_taken_not_missing(self, schema, default_values):
+        data = dict(
+            int_take_default_on_none=None,
+            int_take_default_on_value=0,
+            str_take_default_on_none=None,
+            str_take_default_on_value="bar",
+        )
+        assert schema.dump(data) == default_values
+
+    def test_default_taken_missing(self, schema, default_values):
+        data = dict()
+        assert schema.dump(data) == default_values
+
+    def test_default_not_taken(self, schema, default_values):
+        data = dict(
+            int_take_default_on_none=-1,
+            int_take_default_on_value=-1,
+            str_take_default_on_none="baz",
+            str_take_default_on_value="baz",
+        )
+        assert schema.dump(data) == data
+
+
 class TestDefaults:
     class MySchema(Schema):
         int_no_default = fields.Int(allow_none=True)

tests/test_serialization.py

@@ -135,6 +135,17 @@ def test_integer_field_default_set_to_none(self, user):
         field = fields.Integer(default=None)
         assert field.serialize("age", user) is None
 
+    def test_integer_field_take_default_on(self, user):
+        field = fields.Integer(default=0, take_default_on=[None, 1])
+        user.age = 42
+        assert field.serialize("age", user) == 42
+        del user.age
+        assert field.serialize("age", user) == 0
+        user.age = None
+        assert field.serialize("age", user) == 0
+        user.age = 1
+        assert field.serialize("age", user) == 0
+
     def test_uuid_field(self, user):
         user.uuid1 = uuid.UUID("12345678123456781234567812345678")
         user.uuid2 = None