Fix Constant(None) field rejecting null input

src/marshmallow/fields.py

@@ -2065,10 +2065,13 @@ class Constant(Field[_ContantT]):
     _CHECK_ATTRIBUTE = False
 
     def __init__(self, constant: _ContantT, **kwargs: Unpack[_BaseFieldKwargs]):
+        # Set load_default and dump_default before calling super().__init__
+        # so that allow_none is correctly computed when constant is None
+        # (fixes issue #2868)
+        kwargs.setdefault("load_default", constant)
+        kwargs.setdefault("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

tests/test_deserialization.py

@@ -1413,6 +1413,22 @@ class MySchema(Schema):
         assert sch.load({})["foo"] == 42
         assert sch.load({"foo": 24})["foo"] == 42
 
+    def test_constant_none_allows_null_input(self):
+        """Test that Constant(None) correctly allows null input (issue #2868)."""
+
+        class MySchema(Schema):
+            sentinel = fields.Constant(None)
+
+        schema = MySchema()
+        # Should not raise ValidationError
+        result = schema.load({"sentinel": None})
+        assert result["sentinel"] is None
+        # Should also work with any other input
+        result = schema.load({"sentinel": "anything"})
+        assert result["sentinel"] is None
+        # Dump should also work
+        assert schema.dump({"sentinel": "anything"})["sentinel"] 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"