unittest: Use a recursive type alias for assertIsInstance (#9770)

Author: AlexWaygood

stdlib/builtins.pyi

@@ -1394,6 +1394,7 @@ def iter(__function: Callable[[], _T | None], __sentinel: None) -> Iterator[_T]:
 @overload
 def iter(__function: Callable[[], _T], __sentinel: object) -> Iterator[_T]: ...
 
+# Keep this alias in sync with unittest.case._ClassInfo
 if sys.version_info >= (3, 10):
     _ClassInfo: TypeAlias = type | types.UnionType | tuple[_ClassInfo, ...]
 else:

stdlib/unittest/case.pyi

@@ -68,10 +68,13 @@ class SkipTest(Exception):
 
 class _SupportsAbsAndDunderGE(SupportsDunderGE[Any], SupportsAbs[Any], Protocol): ...
 
+# Keep this alias in sync with builtins._ClassInfo
+# We can't import it from builtins or pytype crashes,
+# due to the fact that pytype uses a custom builtins stub rather than typeshed's builtins stub
 if sys.version_info >= (3, 10):
-    _IsInstanceClassInfo: TypeAlias = type | UnionType | tuple[type | UnionType | tuple[Any, ...], ...]
+    _ClassInfo: TypeAlias = type | UnionType | tuple[_ClassInfo, ...]
 else:
-    _IsInstanceClassInfo: TypeAlias = type | tuple[type | tuple[Any, ...], ...]
+    _ClassInfo: TypeAlias = type | tuple[_ClassInfo, ...]
 
 class TestCase:
     failureException: type[BaseException]
@@ -107,8 +110,8 @@ class TestCase:
     def assertIsNotNone(self, obj: object, msg: Any = None) -> None: ...
     def assertIn(self, member: Any, container: Iterable[Any] | Container[Any], msg: Any = None) -> None: ...
     def assertNotIn(self, member: Any, container: Iterable[Any] | Container[Any], msg: Any = None) -> None: ...
-    def assertIsInstance(self, obj: object, cls: _IsInstanceClassInfo, msg: Any = None) -> None: ...
-    def assertNotIsInstance(self, obj: object, cls: _IsInstanceClassInfo, msg: Any = None) -> None: ...
+    def assertIsInstance(self, obj: object, cls: _ClassInfo, msg: Any = None) -> None: ...
+    def assertNotIsInstance(self, obj: object, cls: _ClassInfo, msg: Any = None) -> None: ...
     @overload
     def assertGreater(self, a: SupportsDunderGT[_T], b: _T, msg: Any = None) -> None: ...
     @overload