Fix Constant field serialization

Author: lafrech

src/marshmallow/fields.py

@@ -2061,6 +2061,9 @@ def __init__(self, constant: _ContantT, **kwargs: Unpack[_BaseFieldKwargs]):
         if kwargs.get("allow_none") is None and constant is None:
             self.allow_none = True
 
+    def _serialize(self, value, **kwargs) -> _ContantT:
+        return self.constant
+
     def _deserialize(self, value, **kwargs) -> _ContantT:
         return self.constant
 

tests/test_serialization.py

@@ -877,7 +877,7 @@ def test_serialize_does_not_apply_validators(self):
 
     def test_constant_field_serialization(self):
         field = fields.Constant("something")
-        assert field.serialize("something") == "something"
+        assert field.serialize("whatever") == "something"
 
     def test_constant_is_always_included_in_serialized_data(self):
         class MySchema(Schema):