Fix @patch when new is missing (#10459)

stdlib/unittest/mock.pyi

@@ -234,6 +234,8 @@ class _patch(Generic[_T]):
     def copy(self) -> _patch[_T]: ...
     @overload
     def __call__(self, func: _TT) -> _TT: ...
+    # If new==DEFAULT, this should add a MagicMock parameter to the function
+    # arguments. See the _patch_default_new class below for this functionality.
     @overload
     def __call__(self, func: Callable[_P, _R]) -> Callable[_P, _R]: ...
     if sys.version_info >= (3, 8):
@@ -257,6 +259,22 @@ class _patch(Generic[_T]):
     def start(self) -> _T: ...
     def stop(self) -> None: ...
 
+if sys.version_info >= (3, 8):
+    _Mock: TypeAlias = MagicMock | AsyncMock
+else:
+    _Mock: TypeAlias = MagicMock
+
+# This class does not exist at runtime, it's a hack to make this work:
+#     @patch("foo")
+#     def bar(..., mock: MagicMock) -> None: ...
+class _patch_default_new(_patch[_Mock]):
+    @overload
+    def __call__(self, func: _TT) -> _TT: ...
+    # Can't use the following as ParamSpec is only allowed as last parameter:
+    #   def __call__(self, func: Callable[_P, _R]) -> Callable[Concatenate[_P, MagicMock], _R]: ...
+    @overload
+    def __call__(self, func: Callable[..., _R]) -> Callable[..., _R]: ...
+
 class _patch_dict:
     in_dict: Any
     values: Any
@@ -273,11 +291,8 @@ class _patch_dict:
     start: Any
     stop: Any
 
-if sys.version_info >= (3, 8):
-    _Mock: TypeAlias = MagicMock | AsyncMock
-else:
-    _Mock: TypeAlias = MagicMock
-
+# This class does not exist at runtime, it's a hack to add methods to the
+# patch() function.
 class _patcher:
     TEST_PREFIX: str
     dict: type[_patch_dict]
@@ -307,7 +322,7 @@ class _patcher:
         autospec: Any | None = ...,
         new_callable: Any | None = ...,
         **kwargs: Any,
-    ) -> _patch[_Mock]: ...
+    ) -> _patch_default_new: ...
     @overload
     @staticmethod
     def object(  # type: ignore[misc]

test_cases/stdlib/check_unittest.py

@@ -5,7 +5,7 @@
 from decimal import Decimal
 from fractions import Fraction
 from typing_extensions import assert_type
-from unittest.mock import Mock, patch
+from unittest.mock import MagicMock, Mock, patch
 
 case = unittest.TestCase()
 
@@ -94,13 +94,20 @@ def __gt__(self, other: Bacon) -> bool:
 ###
 
 
-@patch("sys.exit", new=Mock())
-def f(i: int) -> str:
+@patch("sys.exit")
+def f_default_new(i: int, mock: MagicMock) -> str:
+    return "asdf"
+
+
+@patch("sys.exit", new=42)
+def f_explicit_new(i: int) -> str:
     return "asdf"
 
 
-assert_type(f(1), str)
-f("a")  # type: ignore
+assert_type(f_default_new(1), str)
+f_default_new("a")  # Not an error due to ParamSpec limitations
+assert_type(f_explicit_new(1), str)
+f_explicit_new("a")  # type: ignore[arg-type]
 
 
 @patch("sys.exit", new=Mock())