Wrong exception re-raises if task raises CancelledError inside except block

import asyncio
from contextlib import suppress


async def main():
    task = asyncio.ensure_future(asyncio.sleep(1))
    task.cancel()

    try:
        raise Exception()
    except Exception:
        with suppress(asyncio.CancelledError):
            await task
        raise

if __name__ == "__main__":
    loop = asyncio.get_event_loop()
    loop.run_until_complete(main())

Expected output:

Exception

Actual output:

concurrent.futures._base.CancelledError

Windows 10, Python 3.5.0

I can repro this too -- it's not unique to Windows or to PEP 492 (it fails the same way with coroutine/yield-from in Python 3.4). It's also not due to something in contextlib.suppress() or due to having a try/except in the outer except clause. @1st1 -- do you have any suggestions? I suspect there's some special-casing of CancelledError that goes wrong here.

I might be mistaken here, but it looks like this a bug in CPython...

class MainError(Exception):
    pass

class SubError(Exception):
    pass

def sub():
    yield

def main():
    try:
        raise MainError()
    except MainError:
        try:
            yield from sub()
        except SubError:
            pass
        raise

coro = main()
coro.send(None)
coro.throw(SubError())

Outputs:

Traceback (most recent call last):
  File "t.py", line 47, in <module>
    coro.throw(SubError())
  File "t.py", line 43, in main
    raise
RuntimeError: No active exception to reraise

And if you replace except MainError: with except MainError as e:, and raise with raise e then everything works correctly.

It's not even a yield from bug:

class MainError(Exception):
    pass

class SubError(Exception):
    pass

def main():
    try:
        raise MainError()
    except MainError:
        try:
            yield
        except SubError:
            print('got SubError')
        raise

coro = main()
coro.send(None)
coro.throw(SubError())

Outputs:

got SubError
Traceback (most recent call last):
  File "t.py", line 19, in <module>
    coro.throw(SubError())
  File "t.py", line 15, in main
    raise
RuntimeError: No active exception to reraise

http://bugs.python.org/issue25612

Thanks for the thorough investigation!

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Wrong exception re-raises if task raises CancelledError inside except block #287

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Wrong exception re-raises if task raises CancelledError inside except block #287

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gvanrossum commented on Nov 12, 2015

1st1 commented on Nov 12, 2015

1st1 commented on Nov 12, 2015

1st1 commented on Nov 12, 2015

1st1 commented on Nov 12, 2015

gvanrossum commented on Nov 12, 2015

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