diff --git a/tests/utils.py b/tests/utils.py
index 15fabbc2d0..a0ac11bc5c 100644
--- a/tests/utils.py
+++ b/tests/utils.py
@@ -15,7 +15,7 @@
import atexit
import os
-from typing import Any, Callable, Dict, List, Tuple, Union, overload
+from typing import Any, Callable, Dict, List, Tuple, Type, TypeVar, Union, overload
import attr
from typing_extensions import Literal, ParamSpec
@@ -341,3 +341,27 @@ async def create_room(hs: HomeServer, room_id: str, creator_id: str) -> None:
context = await unpersisted_context.persist(event)
await persistence_store.persist_event(event, context)
+
+
+T = TypeVar("T")
+
+
+def checked_cast(type: Type[T], x: object) -> T:
+ """A version of typing.cast that is checked at runtime.
+
+ We have our own function for this for two reasons:
+
+ 1. typing.cast itself is deliberately a no-op at runtime, see
+ https://docs.python.org/3/library/typing.html#typing.cast
+ 2. To help workaround a mypy-zope bug https://github.com/Shoobx/mypy-zope/issues/91
+ where mypy would erroneously consider `isinstance(x, type)` to be false in all
+ circumstances.
+
+ For this to make sense, `T` needs to be something that `isinstance` can check; see
+ https://docs.python.org/3/library/functions.html?highlight=isinstance#isinstance
+ https://docs.python.org/3/glossary.html#term-abstract-base-class
+ https://docs.python.org/3/library/typing.html#typing.runtime_checkable
+ for more details.
+ """
+ assert isinstance(x, type)
+ return x
|
