✨ Overload `dump`/`dumps` methods to increase type safety;

issue a warning, when using the `many` parameter in `__init__`

src/marshmallow_generic/schema.py

@@ -7,6 +7,7 @@ documentation of [`marshmallow.Schema`][marshmallow.Schema].
 
 from collections.abc import Iterable, Mapping, Sequence
 from typing import TYPE_CHECKING, Any, Literal, Optional, TypeVar, Union, overload
+from warnings import warn
 
 from marshmallow import Schema
 
@@ -43,6 +44,79 @@ class GenericSchema(GenericInsightMixin1[Model], Schema):
     ```
     """
 
+    def __init__(
+        self,
+        *,
+        only: Union[Sequence[str], set[str], None] = None,
+        exclude: Union[Sequence[str], set[str]] = (),
+        context: Union[dict[str, Any], None] = None,
+        load_only: Union[Sequence[str], set[str]] = (),
+        dump_only: Union[Sequence[str], set[str]] = (),
+        partial: Union[bool, Sequence[str], set[str]] = False,
+        unknown: Optional[str] = None,
+        many: Optional[bool] = None,
+    ) -> None:
+        """
+        Emits a warning, if the `many` argument is not `None`.
+
+        Otherwise the same as in [`marshmallow.Schema`][marshmallow.Schema].
+
+        Args:
+            only:
+                Whitelist of the declared fields to select when instantiating
+                the Schema. If `None`, all fields are used. Nested fields can
+                be represented with dot delimiters.
+            exclude:
+                Blacklist of the declared fields to exclude when instantiating
+                the Schema. If a field appears in both `only` and `exclude`,
+                it is not used. Nested fields can be represented with dot
+                delimiters.
+            context:
+                Optional context passed to [`Method`]
+                [marshmallow.fields.Method] and [`Function`]
+                [marshmallow.fields.Function] fields.
+            load_only:
+                Fields to skip during serialization (write-only fields)
+            dump_only:
+                Fields to skip during deserialization (read-only fields)
+            partial:
+                Whether to ignore missing fields and not require any fields
+                declared. Propagates down to [`Nested`]
+                [marshmallow.fields.Nested] fields as well. If its value is an
+                iterable, only missing fields listed in that iterable will be
+                ignored. Use dot delimiters to specify nested fields.
+            unknown:
+                Whether to exclude, include, or raise an error for unknown
+                fields in the data. Use `EXCLUDE`, `INCLUDE` or `RAISE`.
+            many:
+                !!! warning
+                    Specifying this option schema-wide undermines the type
+                    safety that this class aims to provide and passing any
+                    value other than `None` will trigger a warning. Use the
+                    method-specific `many` parameter, when calling
+                    [`dump`][marshmallow_generic.GenericSchema.dump]/
+                    [`dumps`][marshmallow_generic.GenericSchema.dumps] or
+                    [`load`][marshmallow_generic.GenericSchema.load]/
+                    [`loads`][marshmallow_generic.GenericSchema.loads] instead.
+        """
+        if many is not None:
+            warn(
+                "Setting `many` schema-wide breaks type safety. Use the the "
+                "`many` parameter of specific methods (like `load`) instead."
+            )
+        else:
+            many = bool(many)
+        super().__init__(
+            only=only,
+            exclude=exclude,
+            many=many,
+            context=context,
+            load_only=load_only,
+            dump_only=dump_only,
+            partial=partial,
+            unknown=unknown,
+        )
+
     @post_load
     def instantiate(self, data: dict[str, Any], **_kwargs: Any) -> Model:
         """
@@ -52,10 +126,9 @@ class GenericSchema(GenericInsightMixin1[Model], Schema):
         [marshmallow_generic.decorators.post_load] hook for the schema.
 
         !!! warning
-            You should probably **not** use this method directly;
-            no parsing, transformation or validation of any kind is done
-            in this method. The `data` passed to the **`Model`** constructor
-            "as is".
+            You should probably not use this method directly. No parsing,
+            transformation or validation of any kind is done in this method.
+            The `data` is passed to the **`Model`** constructor "as is".
 
         Args:
             data:
@@ -69,6 +142,85 @@ class GenericSchema(GenericInsightMixin1[Model], Schema):
 
     if TYPE_CHECKING:
 
+        @overload  # type: ignore[override]
+        def dump(
+            self,
+            obj: Iterable[Model],
+            *,
+            many: Literal[True],
+        ) -> list[dict[str, Any]]:
+            ...
+
+        @overload
+        def dump(
+            self,
+            obj: Model,
+            *,
+            many: Optional[Literal[False]] = None,
+        ) -> dict[str, Any]:
+            ...
+
+        def dump(
+            self,
+            obj: Union[Model, Iterable[Model]],
+            *,
+            many: Optional[bool] = None,
+        ) -> Union[dict[str, Any], list[dict[str, Any]]]:
+            """
+            Serializes **`Model`** objects to native Python data types.
+
+            Same as [`marshmallow.Schema.dump`]
+            [marshmallow.schema.Schema.dump] at runtime.
+
+            Annotations ensure that type checkers will infer the return type
+            correctly based on the `many` argument, and also enforce the `obj`
+            argument to be an a `list` of **`Model`** instances, if `many` is
+            set to `True` or a single instance of it, if `many` is `False`
+            (or omitted).
+
+            Args:
+                obj:
+                    The object or iterable of objects to serialize
+                many:
+                    Whether to serialize `obj` as a collection. If `None`, the
+                    value for `self.many` is used.
+
+            Returns:
+                (dict[str, Any]): if `many` is set to `False`
+                (list[dict[str, Any]]): if `many` is set to `True`
+            """
+            ...
+
+        @overload  # type: ignore[override]
+        def dumps(
+            self,
+            obj: Iterable[Model],
+            *args: Any,
+            many: Literal[True],
+            **kwargs: Any,
+        ) -> str:
+            ...
+
+        @overload
+        def dumps(
+            self,
+            obj: Model,
+            *args: Any,
+            many: Optional[Literal[False]] = None,
+            **kwargs: Any,
+        ) -> str:
+            ...
+
+        def dumps(
+            self,
+            obj: Union[Model, Iterable[Model]],
+            *args: Any,
+            many: Optional[bool] = None,
+            **kwargs: Any,
+        ) -> str:
+            """Same as [`dump`][marshmallow_generic.GenericSchema.dump], but returns a JSON-encoded string."""
+            ...
+
         @overload  # type: ignore[override]
         def load(
             self,

tests/test_schema.py

@@ -1,3 +1,4 @@
+from typing import Any
 from unittest import TestCase
 from unittest.mock import MagicMock, patch
 
@@ -5,6 +6,29 @@ from marshmallow_generic import _util, schema
 
 
 class GenericSchemaTestCase(TestCase):
+    @patch("marshmallow.schema.Schema.__init__")
+    def test___init__(self, mock_super_init: MagicMock) -> None:
+        class Foo:
+            pass
+
+        kwargs: dict[str, Any] = {
+            "only": object(),
+            "exclude": object(),
+            "context": object(),
+            "load_only": object(),
+            "dump_only": object(),
+            "partial": object(),
+            "unknown": object(),
+            "many": None,
+        }
+        schema.GenericSchema[Foo](**kwargs)
+        mock_super_init.assert_called_once_with(**kwargs | {"many": False})
+        mock_super_init.reset_mock()
+        kwargs["many"] = True
+        with self.assertWarns(UserWarning):
+            schema.GenericSchema[Foo](**kwargs)
+        mock_super_init.assert_called_once_with(**kwargs)
+
     @patch.object(_util.GenericInsightMixin, "_get_type_arg")
     def test_instantiate(self, mock__get_type_arg: MagicMock) -> None:
         mock__get_type_arg.return_value = mock_cls = MagicMock()
@@ -20,6 +44,26 @@ class GenericSchemaTestCase(TestCase):
         mock__get_type_arg.assert_called_once_with(0)
         mock_cls.assert_called_once_with(**mock_data)
 
+    def test_dump_and_dumps(self) -> None:
+        """Mainly for static type checking purposes."""
+
+        class Foo:
+            pass
+
+        class TestSchema(schema.GenericSchema[Foo]):
+            pass
+
+        foo = Foo()
+        single: dict[str, Any] = TestSchema().dump(foo)
+        self.assertDictEqual({}, single)
+        json_string: str = TestSchema().dumps(foo)
+        self.assertEqual("{}", json_string)
+
+        multiple: list[dict[str, Any]] = TestSchema().dump([foo], many=True)
+        self.assertListEqual([{}], multiple)
+        json_string = TestSchema().dumps([foo], many=True)
+        self.assertEqual("[{}]", json_string)
+
     def test_load_and_loads(self) -> None:
         """Mainly for static type checking purposes."""