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Allow to extend return types of functions decorated by ContextDecorator with None #14439

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Allow to extend return types of functions decorated by ContextDecorator with None #14439

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fff6ca5
Implement `_SuppressedExcReturnT`
bswck Jul 22, 2025
6db7d8d
Increase LoB and improve comments
bswck Jul 22, 2025
976351f
Remove `_F` and `_AF` type var
bswck Jul 22, 2025
9dd131f
Nit: remove commas where incorrect
bswck Jul 22, 2025
ed5debc
Improve the comments
bswck Jul 22, 2025
8eefca3
Delegate `_ExcReturnT` to `_GeneratorContextManager` optional type arg
bswck Jul 22, 2025
d604bb2
Add note on type argument inference
bswck Jul 22, 2025
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26 changes: 16 additions & 10 deletions stdlib/contextlib.pyi
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Original file line number Diff line number Diff line change
Expand Up @@ -5,7 +5,7 @@ from abc import ABC, abstractmethod
from collections.abc import AsyncGenerator, AsyncIterator, Awaitable, Callable, Generator, Iterator
from types import TracebackType
from typing import IO, Any, Generic, Protocol, TypeVar, overload, runtime_checkable
from typing_extensions import ParamSpec, Self, TypeAlias
from typing_extensions import Never, ParamSpec, Self, TypeAlias

__all__ = [
"contextmanager",
Expand All @@ -32,9 +32,9 @@ _T = TypeVar("_T")
_T_co = TypeVar("_T_co", covariant=True)
_T_io = TypeVar("_T_io", bound=IO[str] | None)
_ExitT_co = TypeVar("_ExitT_co", covariant=True, bound=bool | None, default=bool | None)
_F = TypeVar("_F", bound=Callable[..., Any])
_G_co = TypeVar("_G_co", bound=Generator[Any, Any, Any] | AsyncGenerator[Any, Any], covariant=True)
_P = ParamSpec("_P")
_R = TypeVar("_R")

_SendT_contra = TypeVar("_SendT_contra", contravariant=True, default=None)
_ReturnT_co = TypeVar("_ReturnT_co", covariant=True, default=None)
Expand Down Expand Up @@ -64,9 +64,16 @@ class AbstractAsyncContextManager(ABC, Protocol[_T_co, _ExitT_co]): # type: ign
self, exc_type: type[BaseException] | None, exc_value: BaseException | None, traceback: TracebackType | None, /
) -> _ExitT_co: ...

class ContextDecorator:
_ExcReturnT = TypeVar("_ExcReturnT", Never, None, default=Never)

# __exit__ can suppress exceptions by returning a true value.
# _ExcReturnT describes function's return type after an exception occurs:
# - Never (default, the context manager never suppresses exceptions)
# - None (the context manager may suppress exceptions)
# See #13512.
class ContextDecorator(Generic[_ExcReturnT]):
def _recreate_cm(self) -> Self: ...
def __call__(self, func: _F) -> _F: ...
def __call__(self, func: Callable[_P, _R]) -> Callable[_P, _R | _ExcReturnT]: ...

class _GeneratorContextManagerBase(Generic[_G_co]):
# Ideally this would use ParamSpec, but that requires (*args, **kwargs), which this isn't. see #6676
Expand All @@ -79,7 +86,7 @@ class _GeneratorContextManagerBase(Generic[_G_co]):
class _GeneratorContextManager(
_GeneratorContextManagerBase[Generator[_T_co, _SendT_contra, _ReturnT_co]],
AbstractContextManager[_T_co, bool | None],
ContextDecorator,
ContextDecorator[_ExcReturnT], # _ExcReturnT is inferred by the type checker
):
def __exit__(
self, typ: type[BaseException] | None, value: BaseException | None, traceback: TracebackType | None
Expand All @@ -88,16 +95,15 @@ class _GeneratorContextManager(
def contextmanager(func: Callable[_P, Iterator[_T_co]]) -> Callable[_P, _GeneratorContextManager[_T_co]]: ...

if sys.version_info >= (3, 10):
_AF = TypeVar("_AF", bound=Callable[..., Awaitable[Any]])

class AsyncContextDecorator:
# _ExcReturnT: see ContextDecorator.
class AsyncContextDecorator(Generic[_ExcReturnT]):
def _recreate_cm(self) -> Self: ...
def __call__(self, func: _AF) -> _AF: ...
def __call__(self, func: Callable[_P, _R]) -> Callable[_P, _R | _ExcReturnT]: ...

class _AsyncGeneratorContextManager(
_GeneratorContextManagerBase[AsyncGenerator[_T_co, _SendT_contra]],
AbstractAsyncContextManager[_T_co, bool | None],
AsyncContextDecorator,
AsyncContextDecorator[_ExcReturnT], # _ExcReturnT is inferred by the type checker
):
async def __aexit__(
self, typ: type[BaseException] | None, value: BaseException | None, traceback: TracebackType | None
Expand Down
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