The following hangs at 100% CPU on Python 3.5, though not on Python 3.4:

1) Start an asyncio coroutine with run_until_complete().
2) Inside the coroutine, enter an ExitStack using a with-statement.
3) Inside the with-statement, call ExitStack.enter_context() with a generator context manager. It doesn't matter what the generator yields.
4) After the enter_context() call, raise an exception.

Here's an example script that does all of this and repros the hang: https://gist.github.com/oconnor663/483db2820bb5f877c9ed

Interestingly, it doesn't hang when you raise a different error. There's some new code dealing with the RuntimeError coming out of a generator if it raises StopIteration (instead of returning) introduced by issue #22906. Yury, it looks like you introduced that? (The diff is this:

changeset:   95932:36a8d935c322
user:        Yury Selivanov <yselivanov@sprymix.com>
date:        Sat May 09 11:44:30 2015 -0400
summary:     PEP 479: Change StopIteration handling inside generators.

Trying to reproduce without contextstack.

Here's a minimal test to reproduce:

    import reprlib


    def main():
        if 0:
            yield
        raise RuntimeError


    m = main()
    try:
        m.send(None)
    except RuntimeError as ex:
        ex.__context__ = ex
        reprlib.repr(ex)
        

Looks like it's a bug in reprlib.  It's not related to PEP 492/479.  

It's also reproducible in Python 3.4 and 3.3.

Nick, ExitStack does this (indirectly) 'ex.__context__ = ex' thing -- I think that's a bug of contextlib.

Created another issue for the reprlib bug: issue 25781.  It appears we don't even need a generator:

    import reprlib

    try:
        raise RuntimeError
    except RuntimeError as ex:
        ex.__context__ = ex
        reprlib.repr(ex)

Closing this one with "not a bug".

It's not even a reprlib bug:

try:
    raise Exception
except Exception as ex:
    ex.__context__ = ex
    hasattr(1, 'aa')

Thanks for chasing this down. Yury, can you suggest a workaround?

> Thanks for chasing this down. Yury, can you suggest a workaround?

I'm not sure how to workaround this :(  Hopefully we can fix this in 3.5.1.

FWIW the bug was identified in issue 25782.  I've drafted a patch to fix it, please review.

The question is whether we should raise an exception or not:

   ex.__context__ = ex

Another issue for contextlib: http://bugs.python.org/issue25786

Yury, can you help me understand why `hasattr("foo", "bar")` triggers the infinite loop there, but not `print("foo")`?

> Yury, can you help me understand why `hasattr("foo", "bar")` triggers the infinite loop there, but not `print("foo")`?


hasattr uses getattr under the hood. getattr raises an AttributeError, and that triggers PyErr_SetError, which has an infinite "while" loop.  Instead of "hasattr" you can use anything that raises an error.