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Commita6f948b

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Fix exception context in @contextmanagers
Previously, when using a `try...yield...except` construct within a`@contextmanager` function, an exception raised by the `yield` wouldn'tbe properly cleared when handled the `except` block. Instead, calling`sys.exception()` would continue to return the exception, leading toconfusing tracebacks and logs.This was happening due to the way that the `@contextmanager` decoratordrives its decorated generator function. When an exception occurs, ituses `.throw(exc)` to throw the exception into the generator. However,even if the generator handles this exception, because the exception wasthrown into it in the context of the `@contextmanager` decoratorhandling it (and not being finished yet), `sys.exception()` was notbeing reset.In order to fix this, the exception context as the `@contextmanager`decorator is `__enter__`'d is stored and set as the current exceptioncontext just before throwing the new exception into the generator.Doing this means that after the generator handles the thrown exception,`sys.exception()` reverts back to what it was when the `@contextmanager`function was started.
1 parentd2f6251 commita6f948b

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3 files changed

+96
-0
lines changed

3 files changed

+96
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‎Lib/contextlib.py

Lines changed: 12 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -106,6 +106,7 @@ class _GeneratorContextManagerBase:
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"""Shared functionality for @contextmanager and @asynccontextmanager."""
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def__init__(self,func,args,kwds):
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self.exc_context=None
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self.gen=func(*args,**kwds)
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self.func,self.args,self.kwds=func,args,kwds
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# Issue 19330: ensure context manager instances have good docstrings
@@ -134,6 +135,8 @@ class _GeneratorContextManager(
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"""Helper for @contextmanager decorator."""
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136137
def__enter__(self):
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# store the exception context on enter so it can be restored on exit
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self.exc_context=sys.exception()
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# do not keep args and kwds alive unnecessarily
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# they are only needed for recreation, which is not possible anymore
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delself.args,self.kwds,self.func
@@ -143,6 +146,9 @@ def __enter__(self):
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raiseRuntimeError("generator didn't yield")fromNone
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def__exit__(self,typ,value,traceback):
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# don't keep the stored exception alive unnecessarily
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exc_context=self.exc_context
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self.exc_context=None
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iftypisNone:
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try:
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next(self.gen)
@@ -159,6 +165,12 @@ def __exit__(self, typ, value, traceback):
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# tell if we get the same exception back
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value=typ()
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try:
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# If the generator handles the exception thrown into it, the
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# exception context will revert to the actual current exception
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# context here. In order to make the context manager behave
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# like a normal function we set the current exception context
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# to what it was during the context manager's __enter__
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sys._set_exception(exc_context)
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self.gen.throw(value)
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exceptStopIterationasexc:
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# Suppress StopIteration *unless* it's the same exception that

‎Lib/test/test_contextlib.py

Lines changed: 82 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -306,6 +306,88 @@ def woohoo():
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withwoohoo():
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raiseStopIteration
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deftest_contextmanager_handling_exception_resets_exc_info(self):
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# Test that sys.exc_info() is correctly unset after handling the error
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# when used within a context manager
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@contextmanager
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defctx(reraise=False):
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try:
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self.assertIsNone(sys.exception())
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yield
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except:
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self.assertIsInstance(sys.exception(),ZeroDivisionError)
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ifreraise:
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raise
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else:
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self.assertIsNone(sys.exception())
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self.assertIsNone(sys.exception())
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withctx():
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pass
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withctx():
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1/0
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withself.assertRaises(ZeroDivisionError):
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withctx(reraise=True):
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1/0
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deftest_contextmanager_while_handling(self):
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# test that any exceptions currently being handled are preserved
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# through the context manager
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@contextmanager
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defctx(reraise=False):
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# called while handling an IndexError --> TypeError
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self.assertIsInstance(sys.exception(),TypeError)
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self.assertIsInstance(sys.exception().__context__,IndexError)
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exc_ctx=sys.exception()
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try:
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# raises a ValueError --> ZeroDivisionError
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yield
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except:
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self.assertIsInstance(sys.exception(),ZeroDivisionError)
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self.assertIsInstance(sys.exception().__context__,ValueError)
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# original error context is preserved
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self.assertIs(sys.exception().__context__.__context__,exc_ctx)
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ifreraise:
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raise
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# inner error handled, context should now be the original context
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self.assertIs(sys.exception(),exc_ctx)
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try:
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raiseIndexError()
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except:
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try:
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raiseTypeError()
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except:
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withctx():
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try:
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raiseValueError()
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except:
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self.assertIsInstance(sys.exception(),ValueError)
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1/0
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self.assertIsInstance(sys.exception(),TypeError)
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self.assertIsInstance(sys.exception(),IndexError)
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try:
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raiseIndexError()
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except:
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try:
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raiseTypeError()
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except:
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withself.assertRaises(ZeroDivisionError):
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withctx(reraise=True):
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try:
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raiseValueError()
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except:
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self.assertIsInstance(sys.exception(),ValueError)
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1/0
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self.assertIsInstance(sys.exception(),TypeError)
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self.assertIsInstance(sys.exception(),IndexError)
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309391
def_create_contextmanager_attribs(self):
310392
defattribs(**kw):
311393
defdecorate(func):
Lines changed: 2 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,2 @@
1+
Fix handling of ``sys.exception()`` within ``@contextlib.contextmanager``
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functions. Patch by Carey Metcalfe.

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