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Fix Constant field rejecting None values during load #2894

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Fix Constant field rejecting None values during load #2894

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4 changes: 2 additions & 2 deletions src/marshmallow/fields.py
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Original file line number Diff line number Diff line change
Expand Up @@ -2065,10 +2065,10 @@ class Constant(Field[_ContantT]):
_CHECK_ATTRIBUTE = False

def __init__(self, constant: _ContantT, **kwargs: Unpack[_BaseFieldKwargs]):
kwargs["load_default"] = constant
kwargs["dump_default"] = constant
super().__init__(**kwargs)
self.constant = constant
self.load_default = constant
self.dump_default = constant

def _serialize(self, value, *args, **kwargs) -> _ContantT:
return self.constant
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9 changes: 9 additions & 0 deletions tests/test_deserialization.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1413,6 +1413,15 @@ class MySchema(Schema):
assert sch.load({})["foo"] == 42
assert sch.load({"foo": 24})["foo"] == 42

def test_constant_none_allows_none_value(self):
class MySchema(Schema):
foo = fields.Constant(None)

sch = MySchema()
assert sch.load({"foo": None})["foo"] is None
assert sch.load({})["foo"] is None
assert sch.load({"foo": "ignored"})["foo"] is None

def test_field_deserialization_with_user_validator_function(self):
field = fields.String(validate=predicate(lambda s: s.lower() == "valid"))
assert field.deserialize("Valid") == "Valid"
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