[3.10] bpo-44594: fix (Async)ExitStack handling of __context__ (gh-27089) (GH-28730)
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..
(cherry picked from commit e6d1aa1ac65b6908fdea2c70ec3aa8c4f1dffcb5)
Co-authored-by: John Belmonte <john@neggie.net>
Automerge-Triggered-By: GH:njsmith
diff --git a/Lib/contextlib.py b/Lib/contextlib.py
index 1e0a39f..c63a849 100644
--- a/Lib/contextlib.py
+++ b/Lib/contextlib.py
@@ -540,10 +540,10 @@ def _fix_exception_context(new_exc, old_exc):
# 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
@@ -674,10 +674,10 @@ def _fix_exception_context(new_exc, old_exc):
# 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 5a08065..fbaae2d 100644
--- a/Lib/test/test_contextlib.py
+++ b/Lib/test/test_contextlib.py
@@ -780,6 +780,40 @@ def suppress_exc(*exc_details):
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 603162e..127d750 100644
--- a/Lib/test/test_contextlib_async.py
+++ b/Lib/test/test_contextlib_async.py
@@ -552,6 +552,41 @@ async def suppress_exc(*exc_details):
self.assertIsInstance(inner_exc, ValueError)
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")
+
class TestAsyncNullcontext(unittest.TestCase):
@_async_test
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.