From e6d1aa1ac65b6908fdea2c70ec3aa8c4f1dffcb5 Mon Sep 17 00:00:00 2001 From: John Belmonte Date: Mon, 4 Oct 2021 15:49:55 +0900 Subject: bpo-44594: fix (Async)ExitStack handling of __context__ (gh-27089) * bpo-44594: fix (Async)ExitStack handling of __context__ Make enter_context(foo()) / enter_async_context(foo()) equivalent to `[async] with foo()` regarding __context__ when an exception is raised. Previously exceptions would be caught and re-raised with the wrong context when explicitly overriding __context__ with None. --- Lib/contextlib.py | 8 ++--- Lib/test/test_contextlib.py | 34 +++++++++++++++++++++ Lib/test/test_contextlib_async.py | 35 ++++++++++++++++++++++ .../2021-07-12-10-32-48.bpo-44594.eEa5zi.rst | 3 ++ 4 files changed, 76 insertions(+), 4 deletions(-) create mode 100644 Misc/NEWS.d/next/Library/2021-07-12-10-32-48.bpo-44594.eEa5zi.rst diff --git a/Lib/contextlib.py b/Lib/contextlib.py index 1384d89..d90ca5d 100644 --- a/Lib/contextlib.py +++ b/Lib/contextlib.py @@ -553,10 +553,10 @@ class ExitStack(_BaseExitStack, AbstractContextManager): # Context may not be correct, so find the end of the chain while 1: exc_context = new_exc.__context__ - if exc_context is old_exc: + if exc_context is None or exc_context is old_exc: # Context is already set correctly (see issue 20317) return - if exc_context is None or exc_context is frame_exc: + if exc_context is frame_exc: break new_exc = exc_context # Change the end of the chain to point to the exception @@ -693,10 +693,10 @@ class AsyncExitStack(_BaseExitStack, AbstractAsyncContextManager): # Context may not be correct, so find the end of the chain while 1: exc_context = new_exc.__context__ - if exc_context is old_exc: + if exc_context is None or exc_context is old_exc: # Context is already set correctly (see issue 20317) return - if exc_context is None or exc_context is frame_exc: + if exc_context is frame_exc: break new_exc = exc_context # Change the end of the chain to point to the exception diff --git a/Lib/test/test_contextlib.py b/Lib/test/test_contextlib.py index 43b8507..7982d9d 100644 --- a/Lib/test/test_contextlib.py +++ b/Lib/test/test_contextlib.py @@ -799,6 +799,40 @@ class TestBaseExitStack: self.assertIsInstance(inner_exc, ValueError) self.assertIsInstance(inner_exc.__context__, ZeroDivisionError) + def test_exit_exception_explicit_none_context(self): + # Ensure ExitStack chaining matches actual nested `with` statements + # regarding explicit __context__ = None. + + class MyException(Exception): + pass + + @contextmanager + def my_cm(): + try: + yield + except BaseException: + exc = MyException() + try: + raise exc + finally: + exc.__context__ = None + + @contextmanager + def my_cm_with_exit_stack(): + with self.exit_stack() as stack: + stack.enter_context(my_cm()) + yield stack + + for cm in (my_cm, my_cm_with_exit_stack): + with self.subTest(): + try: + with cm(): + raise IndexError() + except MyException as exc: + self.assertIsNone(exc.__context__) + else: + self.fail("Expected IndexError, but no exception was raised") + def test_exit_exception_non_suppressing(self): # http://bugs.python.org/issue19092 def raise_exc(exc): diff --git a/Lib/test/test_contextlib_async.py b/Lib/test/test_contextlib_async.py index c738bf3..c16c7ec 100644 --- a/Lib/test/test_contextlib_async.py +++ b/Lib/test/test_contextlib_async.py @@ -647,6 +647,41 @@ class TestAsyncExitStack(TestBaseExitStack, unittest.TestCase): self.assertIsInstance(inner_exc.__context__, ZeroDivisionError) @_async_test + async def test_async_exit_exception_explicit_none_context(self): + # Ensure AsyncExitStack chaining matches actual nested `with` statements + # regarding explicit __context__ = None. + + class MyException(Exception): + pass + + @asynccontextmanager + async def my_cm(): + try: + yield + except BaseException: + exc = MyException() + try: + raise exc + finally: + exc.__context__ = None + + @asynccontextmanager + async def my_cm_with_exit_stack(): + async with self.exit_stack() as stack: + await stack.enter_async_context(my_cm()) + yield stack + + for cm in (my_cm, my_cm_with_exit_stack): + with self.subTest(): + try: + async with cm(): + raise IndexError() + except MyException as exc: + self.assertIsNone(exc.__context__) + else: + self.fail("Expected IndexError, but no exception was raised") + + @_async_test async def test_instance_bypass_async(self): class Example(object): pass cm = Example() diff --git a/Misc/NEWS.d/next/Library/2021-07-12-10-32-48.bpo-44594.eEa5zi.rst b/Misc/NEWS.d/next/Library/2021-07-12-10-32-48.bpo-44594.eEa5zi.rst new file mode 100644 index 0000000..a2bfd8f --- /dev/null +++ b/Misc/NEWS.d/next/Library/2021-07-12-10-32-48.bpo-44594.eEa5zi.rst @@ -0,0 +1,3 @@ +Fix an edge case of :class:`ExitStack` and :class:`AsyncExitStack` exception +chaining. They will now match ``with`` block behavior when ``__context__`` is +explicitly set to ``None`` when the exception is in flight. -- cgit v0.12