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gh-111375: Fix handling of exceptions within@contextmanager-decorated functions#111676

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14 changes: 13 additions & 1 deletionLib/contextlib.py
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Original file line numberDiff line numberDiff line change
Expand Up@@ -106,6 +106,7 @@ class _GeneratorContextManagerBase:
"""Shared functionality for @contextmanager and @asynccontextmanager."""

def __init__(self, func, args, kwds):
self.exc_context = None
self.gen = func(*args, **kwds)
self.func, self.args, self.kwds = func, args, kwds
# Issue 19330: ensure context manager instances have good docstrings
Expand DownExpand Up@@ -134,6 +135,8 @@ class _GeneratorContextManager(
"""Helper for @contextmanager decorator."""

def __enter__(self):
# store the exception context on enter so it can be restored on exit
self.exc_context = sys.exception()
# do not keep args and kwds alive unnecessarily
# they are only needed for recreation, which is not possible anymore
del self.args, self.kwds, self.func
Expand All@@ -143,6 +146,9 @@ def __enter__(self):
raise RuntimeError("generator didn't yield") from None

def __exit__(self, typ, value, traceback):
# don't keep the stored exception alive unnecessarily
exc_context = self.exc_context
self.exc_context = None
if typ is None:
try:
next(self.gen)
Expand All@@ -159,7 +165,13 @@ def __exit__(self, typ, value, traceback):
# tell if we get the same exception back
value = typ()
try:
self.gen.throw(value)
# If the generator handles the exception thrown into it, the
# exception context reverts to the actual current exception
# context here. In order to make the context manager behave
# like a normal function we set the current exception context
# to what it was during the context manager's __enter__
# (see gh-111676).
self.gen.throw(value, exc_context=exc_context)
except StopIteration as exc:
# Suppress StopIteration *unless* it's the same exception that
# was passed to throw(). This prevents a StopIteration
Expand Down
92 changes: 92 additions & 0 deletionsLib/test/test_contextlib.py
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Original file line numberDiff line numberDiff line change
Expand Up@@ -306,6 +306,98 @@ def woohoo():
with woohoo():
raise StopIteration

def test_contextmanager_handling_exception_resets_exc_info(self):
# Test that sys.exc_info() is correctly unset after handling the error
# when used within a context manager

@contextmanager
def ctx(reraise=False):
try:
self.assertIsNone(sys.exception())
yield
except:
self.assertIsInstance(sys.exception(), ZeroDivisionError)
if reraise:
raise
else:
self.assertIsNone(sys.exception())
self.assertIsNone(sys.exception())

with ctx():
pass

with ctx():
1/0

with self.assertRaises(ZeroDivisionError):
with ctx(reraise=True):
1/0

def test_contextmanager_while_handling(self):
# test that any exceptions currently being handled are preserved
# through the context manager

@contextmanager
def ctx(reraise=False):
# called while handling an IndexError --> TypeError
self.assertIsInstance(sys.exception(), TypeError)
self.assertIsInstance(sys.exception().__context__, IndexError)
exc_ctx = sys.exception()
try:
# raises a ValueError --> ZeroDivisionError
yield
except:
self.assertIsInstance(sys.exception(), ZeroDivisionError)
self.assertIsInstance(sys.exception().__context__, ValueError)
# original error context is preserved
self.assertIs(sys.exception().__context__.__context__, exc_ctx)
if reraise:
raise

# inner error handled, context should now be the original context
self.assertIs(sys.exception(), exc_ctx)

try:
raise IndexError()
except:
try:
raise TypeError()
except:
with ctx():
self.assertIsInstance(sys.exception(), TypeError)
self.assertIsInstance(sys.exception().__context__, IndexError)
try:
raise ValueError()
except:
self.assertIsInstance(sys.exception(), ValueError)
self.assertIsInstance(sys.exception().__context__, TypeError)
self.assertIsInstance(sys.exception().__context__.__context__, IndexError)
1/0
self.assertIsInstance(sys.exception(), TypeError)
self.assertIsInstance(sys.exception().__context__, IndexError)
self.assertIsInstance(sys.exception(), IndexError)

try:
raise IndexError()
except:
try:
raise TypeError()
except:
with self.assertRaises(ZeroDivisionError):
with ctx(reraise=True):
self.assertIsInstance(sys.exception(), TypeError)
self.assertIsInstance(sys.exception().__context__, IndexError)
try:
raise ValueError()
except:
self.assertIsInstance(sys.exception(), ValueError)
self.assertIsInstance(sys.exception().__context__, TypeError)
self.assertIsInstance(sys.exception().__context__.__context__, IndexError)
1/0
self.assertIsInstance(sys.exception(), TypeError)
self.assertIsInstance(sys.exception().__context__, IndexError)
self.assertIsInstance(sys.exception(), IndexError)

def _create_contextmanager_attribs(self):
def attribs(**kw):
def decorate(func):
Expand Down
125 changes: 125 additions & 0 deletionsLib/test/test_sys.py
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Original file line numberDiff line numberDiff line change
Expand Up@@ -161,6 +161,131 @@ def f():
self.assertIsInstance(e, ValueError)
self.assertIs(exc, e)

class SetExceptionTests(unittest.TestCase):

def tearDown(self):
# make sure we don't leave the global exception set
sys._set_exception(None);

def test_set_exc_invalid_values(self):
for x in (0, "1", b"2"):
with self.assertRaises(TypeError):
sys._set_exception(x);

def test_clear_exc(self):
try:
raise ValueError()
except ValueError:
self.assertIsInstance(sys.exception(), ValueError)
sys._set_exception(None)
self.assertIsNone(sys.exception())

def test_set_exc(self):
exc = ValueError()
self.assertIsNone(sys.exception())
sys._set_exception(exc)
self.assertIs(sys.exception(), exc)

def test_set_exc_replaced_by_new_exception_and_restored(self):
exc = ValueError()
sys._set_exception(exc)
self.assertIs(sys.exception(), exc)
try:
raise TypeError()
except TypeError:
self.assertIsInstance(sys.exception(), TypeError)
self.assertIs(sys.exception().__context__, exc)

self.assertIs(sys.exception(), exc)

def test_set_exc_popped_on_exit_except(self):
exc = ValueError()
try:
raise TypeError()
except TypeError:
self.assertIsInstance(sys.exception(), TypeError)
sys._set_exception(exc)
self.assertIs(sys.exception(), exc)
self.assertIsNone(sys.exception())

def test_cleared_exc_overridden_and_restored(self):
try:
raise ValueError()
except ValueError:
try:
raise TypeError()
except TypeError:
self.assertIsInstance(sys.exception(), TypeError)
sys._set_exception(None)
self.assertIsNone(sys.exception())
try:
raise IndexError()
except IndexError:
self.assertIsInstance(sys.exception(), IndexError)
self.assertIsNone(sys.exception().__context__)
self.assertIsNone(sys.exception())
self.assertIsInstance(sys.exception(), ValueError)
self.assertIsNone(sys.exception())

def test_clear_exc_in_generator(self):
def inner():
self.assertIsNone(sys.exception())
yield
self.assertIsInstance(sys.exception(), ValueError)
sys._set_exception(None)
self.assertIsNone(sys.exception())
yield
self.assertIsNone(sys.exception())

# with a single exception in exc_info stack
g = inner()
next(g)
try:
raise ValueError()
except:
self.assertIsInstance(sys.exception(), ValueError)
next(g)
self.assertIsInstance(sys.exception(), ValueError)
self.assertIsNone(sys.exception())
with self.assertRaises(StopIteration):
next(g)
self.assertIsNone(sys.exception())

# with multiple exceptions in exc_info stack by chaining generators
def outer():
g = inner()
self.assertIsNone(sys.exception())
yield next(g)
self.assertIsInstance(sys.exception(), TypeError)
try:
raise ValueError()
except:
self.assertIsInstance(sys.exception(), ValueError)
self.assertIsInstance(sys.exception().__context__, TypeError)
yield next(g)
# at this point the TypeError from the caller has been handled
# by the caller's except block. Even still, it should still be
# referenced as the __context__ of the current exception.
self.assertIsInstance(sys.exception(), ValueError)
self.assertIsInstance(sys.exception().__context__, TypeError)
# not handling an exception, caller isn't handling one either
self.assertIsNone(sys.exception())
with self.assertRaises(StopIteration):
next(g)
self.assertIsNone(sys.exception())

g = outer()
next(g)
try:
raise TypeError()
except:
self.assertIsInstance(sys.exception(), TypeError)
next(g)
self.assertIsInstance(sys.exception(), TypeError)
self.assertIsNone(sys.exception())
with self.assertRaises(StopIteration):
next(g)
self.assertIsNone(sys.exception())

class ExceptHookTest(unittest.TestCase):

Expand Down
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Original file line numberDiff line numberDiff line change
@@ -0,0 +1,2 @@
Add sys._set_exception() function that can set/clear the current exception
context. Patch by Carey Metcalfe.
View file
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Original file line numberDiff line numberDiff line change
@@ -0,0 +1,2 @@
Fix handling of ``sys.exception()`` within ``@contextlib.contextmanager``
functions. Patch by Carey Metcalfe.
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