fix(2549): Ensure MultiDict and ImmutableMultiDict copy methods r…

…eturn the instance's type (#3009)

* Fix MultiDict and ImmutableMultiDict copy methods

* Fix typing

* Fix typing

litestar/datastructures/multi_dicts.py

@@ -1,13 +1,17 @@
 from __future__ import annotations
 
 from abc import ABC
-from typing import Any, Generator, Generic, Iterable, Mapping, TypeVar
+from typing import TYPE_CHECKING, Any, Generator, Generic, Iterable, Mapping, TypeVar
 
 from multidict import MultiDict as BaseMultiDict
 from multidict import MultiDictProxy, MultiMapping
 
 from litestar.datastructures.upload_file import UploadFile
 
+if TYPE_CHECKING:
+    from typing_extensions import Self
+
+
 __all__ = ("FormMultiDict", "ImmutableMultiDict", "MultiDict", "MultiMixin")
 
 
@@ -40,7 +44,8 @@ class MultiDict(BaseMultiDict[T], MultiMixin[T], Generic[T]):
     """MultiDict, using :class:`MultiDict <multidict.MultiDictProxy>`."""
 
     def __init__(self, args: MultiMapping | Mapping[str, T] | Iterable[tuple[str, T]] | None = None) -> None:
-        """Initialize ``MultiDict`` from a`MultiMapping``, :class:`Mapping <typing.Mapping>` or an iterable of tuples.
+        """Initialize ``MultiDict`` from a`MultiMapping``,
+        :class:`Mapping <typing.Mapping>` or an iterable of tuples.
 
         Args:
             args: Mapping-like structure to create the ``MultiDict`` from
@@ -57,30 +62,35 @@ def immutable(self) -> ImmutableMultiDict[T]:
         """
         return ImmutableMultiDict[T](self)  # pyright: ignore
 
+    def copy(self) -> Self:
+        """Return a shallow copy"""
+        return type(self)(list(self.multi_items()))
+
 
 class ImmutableMultiDict(MultiDictProxy[T], MultiMixin[T], Generic[T]):
     """Immutable MultiDict, using class:`MultiDictProxy <multidict.MultiDictProxy>`."""
 
     def __init__(self, args: MultiMapping | Mapping[str, Any] | Iterable[tuple[str, Any]] | None = None) -> None:
-        """Initialize ``ImmutableMultiDict`` from a.
-
-        ``MultiMapping``, :class:`Mapping <typing.Mapping>` or an iterable of tuples.
+        """Initialize ``ImmutableMultiDict`` from a `MultiMapping``,
+        :class:`Mapping <typing.Mapping>` or an iterable of tuples.
 
         Args:
             args: Mapping-like structure to create the ``ImmutableMultiDict`` from
         """
         super().__init__(BaseMultiDict(args or {}))
 
     def mutable_copy(self) -> MultiDict[T]:
-        """Create a mutable copy as a.
-
-        :class:`MultiDict`
+        """Create a mutable copy as a :class:`MultiDict`
 
         Returns:
             A mutable multi dict
         """
         return MultiDict(list(self.multi_items()))
 
+    def copy(self) -> Self:  # type: ignore[override]
+        """Return a shallow copy"""
+        return type(self)(self.items())
+
 
 class FormMultiDict(ImmutableMultiDict[Any]):
     """MultiDict for form data."""

tests/unit/test_datastructures/test_multi_dicts.py

@@ -1,4 +1,4 @@
-from typing import Type, Union
+from __future__ import annotations
 
 import pytest
 from pytest_mock import MockerFixture
@@ -8,14 +8,14 @@
 
 
 @pytest.mark.parametrize("multi_class", [MultiDict, ImmutableMultiDict])
-def test_multi_to_dict(multi_class: Type[Union[MultiDict, ImmutableMultiDict]]) -> None:
+def test_multi_to_dict(multi_class: type[MultiDict | ImmutableMultiDict]) -> None:
     multi = multi_class([("key", "value"), ("key", "value2"), ("key2", "value3")])
 
     assert multi.dict() == {"key": ["value", "value2"], "key2": ["value3"]}
 
 
 @pytest.mark.parametrize("multi_class", [MultiDict, ImmutableMultiDict])
-def test_multi_multi_items(multi_class: Type[Union[MultiDict, ImmutableMultiDict]]) -> None:
+def test_multi_multi_items(multi_class: type[MultiDict | ImmutableMultiDict]) -> None:
     data = [("key", "value"), ("key", "value2"), ("key2", "value3")]
     multi = multi_class(data)
 
@@ -47,3 +47,11 @@ async def test_form_multi_dict_close(mocker: MockerFixture) -> None:
     await multi.close()
 
     assert close.call_count == 2
+
+
+@pytest.mark.parametrize("type_", [MultiDict, ImmutableMultiDict])
+def test_copy(type_: type[MultiDict | ImmutableMultiDict]) -> None:
+    d = type_([("foo", "bar"), ("foo", "baz")])
+    copy = d.copy()
+    assert set(d.multi_items()) == set(copy.multi_items())
+    assert isinstance(d, type_)