Bug report

Bug description:

The issue

I would expect that handling exceptions within acontextlib.contextmanager-created function would work in the same way as other functions and clear thesys.exception() after an error is handled.

importcontextlibimportsysdefp(msg):print(msg,repr(sys.exception()),sep=": ")defctx_gen():p("before yield")try:yieldexcept:p("during handling")p("after handling")ctx=contextlib.contextmanager(ctx_gen)withctx():1/0

The above example prints:

before yield: Noneduring handling: ZeroDivisionError('division by zero')after handling: ZeroDivisionError('division by zero')

Whereas since the error was handled by theexcept: block, my expectation was:

before yield: Noneduring handling: ZeroDivisionError('division by zero')after handling: None

Just working as designed?

From doing some digging, it seems like this is happening because the exception is still being handled by the_GeneratorContextManager.__exit__ function (added by the@contextlib.contextmanager decorator) that's driving thectx_gen generator, even after thectg_gen has handled it.

The following is a very rough approximation of how@contextlib.contextmanager drivesctx_gen:

c=ctx_gen()next(c)# __enter__()try:# code inside the with block1/0exceptExceptionase:# __exit__(typ, exc, tb) for e# throw exception into generator and expect to run to endtry:c.throw(e)exceptStopIteration:passelse:# __exit__(None, None, None)# expect to run to endtry:next(e)exceptStopIteration:pass

Running the above (including the definitions from the first code block) also prints:

before yield: Noneduring handling: ZeroDivisionError('division by zero')after handling: ZeroDivisionError('division by zero')

In the above code, it's more clear that the exception is still being handled by theexcept Exception as e: block untilc.throw() returns/raises, which only happens after the generator exits. Therefore the exception is still being handled the entire timectx_gen is running all the code after the firstyield.

The fix?

Even though this behavior looks to be technically correct, it still seems unexpected and a bit of an abstraction leak.

Is this something that can be/should be fixed? Or should the behavior just be documented?

CPython versions tested on:

3.8, 3.9, 3.10, 3.11, CPython main branch

Operating systems tested on:

macOS

Linked PRs

• gh-111375: Fix handling of exceptions within@contextmanager-decorated functions #111676
• gh-111375: UseNULL in the exception stack to indicate an exception was handled #113302