add field_serializer to computed_field (#6965)

Co-authored-by: Adrian Garcia Badaracco <1755071+adriangb@users.noreply.github.com>

pydantic/_internal/_decorators.py

@@ -527,7 +527,9 @@ def inspect_validator(validator: Callable[..., Any], mode: FieldValidatorModes)
     )
 
 
-def inspect_field_serializer(serializer: Callable[..., Any], mode: Literal['plain', 'wrap']) -> tuple[bool, bool]:
+def inspect_field_serializer(
+    serializer: Callable[..., Any], mode: Literal['plain', 'wrap'], computed_field: bool = False
+) -> tuple[bool, bool]:
     """Look at a field serializer function and determine if it is a field serializer,
     and whether it takes an info argument.
 
@@ -536,6 +538,8 @@ def inspect_field_serializer(serializer: Callable[..., Any], mode: Literal['plai
     Args:
         serializer: The serializer function to inspect.
         mode: The serializer mode, either 'plain' or 'wrap'.
+        computed_field: When serializer is applied on computed_field. It doesn't require
+            info signature.
 
     Returns:
         Tuple of (is_field_serializer, info_arg).
@@ -557,6 +561,11 @@ def inspect_field_serializer(serializer: Callable[..., Any], mode: Literal['plai
             f'Unrecognized field_serializer function signature for {serializer} with `mode={mode}`:{sig}',
             code='field-serializer-signature',
         )
+    if info_arg and computed_field:
+        raise PydanticUserError(
+            'field_serializer on computed_field does not use info signature', code='field-serializer-signature'
+        )
+
     else:
         return is_field_serializer, info_arg
 

pydantic/_internal/_generate_schema.py

@@ -464,13 +464,14 @@ def _model_schema(self, cls: type[BaseModel]) -> core_schema.CoreSchema:
 
             fields = cls.model_fields
             decorators = cls.__pydantic_decorators__
+            computed_fields = decorators.computed_fields
             check_decorator_fields_exist(
                 chain(
                     decorators.field_validators.values(),
                     decorators.field_serializers.values(),
                     decorators.validators.values(),
                 ),
-                fields.keys(),
+                {*fields.keys(), *computed_fields.keys()},
             )
             config_wrapper = ConfigWrapper(cls.model_config, check=False)
             core_config = config_wrapper.core_config(cls)
@@ -515,11 +516,13 @@ def _model_schema(self, cls: type[BaseModel]) -> core_schema.CoreSchema:
                 else:
                     fields_schema: core_schema.CoreSchema = core_schema.model_fields_schema(
                         {k: self._generate_md_field_schema(k, v, decorators) for k, v in fields.items()},
-                        computed_fields=[self._computed_field_schema(d) for d in decorators.computed_fields.values()],
+                        computed_fields=[
+                            self._computed_field_schema(d, decorators.field_serializers)
+                            for d in computed_fields.values()
+                        ],
                         extra_validator=extra_validator,
                         model_name=cls.__name__,
                     )
-
                     inner_schema = apply_validators(fields_schema, decorators.root_validators.values(), None)
                     inner_schema = define_expected_missing_refs(inner_schema, recursively_defined_type_refs())
                     inner_schema = apply_model_validators(inner_schema, model_validators, 'inner')
@@ -1083,7 +1086,10 @@ def _typed_dict_schema(self, typed_dict_cls: Any, origin: Any) -> core_schema.Co
 
                 td_schema = core_schema.typed_dict_schema(
                     fields,
-                    computed_fields=[self._computed_field_schema(d) for d in decorators.computed_fields.values()],
+                    computed_fields=[
+                        self._computed_field_schema(d, decorators.field_serializers)
+                        for d in decorators.computed_fields.values()
+                    ],
                     ref=typed_dict_ref,
                     metadata=metadata,
                     config=core_config,
@@ -1316,7 +1322,10 @@ def _dataclass_schema(
                 args_schema = core_schema.dataclass_args_schema(
                     dataclass.__name__,
                     args,
-                    computed_fields=[self._computed_field_schema(d) for d in decorators.computed_fields.values()],
+                    computed_fields=[
+                        self._computed_field_schema(d, decorators.field_serializers)
+                        for d in decorators.computed_fields.values()
+                    ],
                     collect_init_only=has_post_init,
                 )
 
@@ -1402,7 +1411,11 @@ def _unsubstituted_typevar_schema(self, typevar: typing.TypeVar) -> core_schema.
         else:
             return core_schema.any_schema()
 
-    def _computed_field_schema(self, d: Decorator[ComputedFieldInfo]) -> core_schema.ComputedField:
+    def _computed_field_schema(
+        self,
+        d: Decorator[ComputedFieldInfo],
+        field_serializers: dict[str, Decorator[FieldSerializerDecoratorInfo]],
+    ) -> core_schema.ComputedField:
         try:
             return_type = _decorators.get_function_return_type(d.func, d.info.return_type, self._types_namespace)
         except NameError as e:
@@ -1415,7 +1428,12 @@ def _computed_field_schema(self, d: Decorator[ComputedFieldInfo]) -> core_schema
             )
 
         return_type_schema = self.generate_schema(return_type)
-
+        # Apply serializers to computed field if there exist
+        return_type_schema = self._apply_field_serializers(
+            return_type_schema,
+            filter_field_decorator_info_by_field(field_serializers.values(), d.cls_var_name),
+            computed_field=True,
+        )
         # Handle alias_generator using similar logic to that from
         # pydantic._internal._generate_schema.GenerateSchema._common_field_schema,
         # with field_info -> d.info and name -> d.cls_var_name
@@ -1677,7 +1695,10 @@ def new_handler(source: Any) -> core_schema.CoreSchema:
         return CallbackGetCoreSchemaHandler(new_handler, self)
 
     def _apply_field_serializers(
-        self, schema: core_schema.CoreSchema, serializers: list[Decorator[FieldSerializerDecoratorInfo]]
+        self,
+        schema: core_schema.CoreSchema,
+        serializers: list[Decorator[FieldSerializerDecoratorInfo]],
+        computed_field: bool = False,
     ) -> core_schema.CoreSchema:
         """Apply field serializers to a schema."""
         if serializers:
@@ -1693,7 +1714,9 @@ def _apply_field_serializers(
 
             # use the last serializer to make it easy to override a serializer set on a parent model
             serializer = serializers[-1]
-            is_field_serializer, info_arg = inspect_field_serializer(serializer.func, serializer.info.mode)
+            is_field_serializer, info_arg = inspect_field_serializer(
+                serializer.func, serializer.info.mode, computed_field=computed_field
+            )
 
             try:
                 return_type = _decorators.get_function_return_type(

tests/test_computed_fields.py

@@ -15,6 +15,7 @@
     TypeAdapter,
     computed_field,
     dataclasses,
+    field_serializer,
     field_validator,
 )
 from pydantic.alias_generators import to_camel
@@ -119,14 +120,23 @@ class Square(BaseModel):
         def area(self) -> float:
             return self.side**2
 
+        @computed_field
+        @property
+        def area_string(self) -> str:
+            return f'{self.area} square units'
+
+        @field_serializer('area_string')
+        def serialize_area_string(self, area_string):
+            return area_string.upper()
+
         @area.setter
         def area(self, new_area: int):
             self.side = new_area**0.5
 
     s = Square(side=10)
-    assert s.model_dump() == {'side': 10.0, 'area': 100.0}
+    assert s.model_dump() == {'side': 10.0, 'area': 100.0, 'area_string': '100.0 SQUARE UNITS'}
     s.area = 64
-    assert s.model_dump() == {'side': 8.0, 'area': 64.0}
+    assert s.model_dump() == {'side': 8.0, 'area': 64.0, 'area_string': '64.0 SQUARE UNITS'}
 
 
 def test_computed_fields_del():

tests/test_serialize.py

@@ -15,6 +15,7 @@
     BaseModel,
     Field,
     FieldSerializationInfo,
+    PydanticUserError,
     SecretStr,
     SerializationInfo,
     SerializeAsAny,
@@ -985,6 +986,98 @@ def two_x(self) -> 'IntAlias':  # noqa F821
     assert Model(x=1).model_dump() == {'two_x': 2, 'x': 1}
 
 
+def test_computed_field_custom_serializer():
+    class Model(BaseModel):
+        x: int
+
+        @computed_field
+        @property
+        def two_x(self) -> int:
+            return self.x * 2
+
+        @field_serializer('two_x', when_used='json')
+        def ser_two_x(self, v):
+            return f'The double of x is {v}'
+
+    m = Model(x=1)
+
+    assert m.model_dump() == {'two_x': 2, 'x': 1}
+    assert json.loads(m.model_dump_json()) == {'two_x': 'The double of x is 2', 'x': 1}
+
+
+def test_annotated_computed_field_custom_serializer():
+    class Model(BaseModel):
+        x: int
+
+        @computed_field
+        @property
+        def two_x(self) -> Annotated[int, PlainSerializer(lambda v: f'The double of x is {v}', return_type=str)]:
+            return self.x * 2
+
+        @computed_field
+        @property
+        def triple_x(self) -> Annotated[int, PlainSerializer(lambda v: f'The triple of x is {v}', return_type=str)]:
+            return self.two_x * 3
+
+        @computed_field
+        @property
+        def quadruple_x_plus_one(self) -> Annotated[int, PlainSerializer(lambda v: v + 1, return_type=int)]:
+            return self.two_x * 2
+
+    m = Model(x=1)
+    assert m.x == 1
+    assert m.two_x == 2
+    assert m.triple_x == 6
+    assert m.quadruple_x_plus_one == 4
+
+    # insert_assert(m.model_dump())
+    assert m.model_dump() == {
+        'x': 1,
+        'two_x': 'The double of x is 2',
+        'triple_x': 'The triple of x is 6',
+        'quadruple_x_plus_one': 5,
+    }
+
+    # insert_assert(json.loads(m.model_dump_json()))
+    assert json.loads(m.model_dump_json()) == {
+        'x': 1,
+        'two_x': 'The double of x is 2',
+        'triple_x': 'The triple of x is 6',
+        'quadruple_x_plus_one': 5,
+    }
+
+    # insert_assert(Model.model_json_schema(mode='serialization'))
+    assert Model.model_json_schema(mode='serialization') == {
+        'properties': {
+            'x': {'title': 'X', 'type': 'integer'},
+            'two_x': {'readOnly': True, 'title': 'Two X', 'type': 'string'},
+            'triple_x': {'readOnly': True, 'title': 'Triple X', 'type': 'string'},
+            'quadruple_x_plus_one': {'readOnly': True, 'title': 'Quadruple X Plus One', 'type': 'integer'},
+        },
+        'required': ['x', 'two_x', 'triple_x', 'quadruple_x_plus_one'],
+        'title': 'Model',
+        'type': 'object',
+    }
+
+
+def test_computed_field_custom_serializer_bad_signature():
+    error_msg = 'field_serializer on computed_field does not use info signature'
+
+    with pytest.raises(PydanticUserError, match=error_msg):
+
+        class Model(BaseModel):
+            x: int
+
+            @computed_field
+            @property
+            def two_x(self) -> int:
+                return self.x * 2
+
+            @field_serializer('two_x')
+            def ser_two_x_bad_signature(self, v, _info):
+                return f'The double of x is {v}'
+
+
 @pytest.mark.skipif(sys.version_info < (3, 9), reason='@computed_field @classmethod @property only works in 3.9+')
 def test_forward_ref_for_classmethod_computed_fields():
     class Model(BaseModel):