async_bug.py

# Triggers a bug related to coroutines and stack frames.
# Run this in Python 3:
# $ python3 async_bug.py [bad]
# When the commandline argument "bad" is passed in, the bug is triggered, and the foo() coroutine never completes.
#

import asyncio
import sys

LOG = print
ev = asyncio.Event()
exn = None
foo_task = None

async def foo():
    global exn
    try:
        LOG("foo: raising exception")
        raise TypeError("blah")
    except TypeError as e:
        exn = e
    LOG("foo: waiting for ev")
    await ev.wait()
    LOG("foo: ev set, quitting now.")
    return "quit successfully"

async def bar(bad):
    await asyncio.sleep(1)
    LOG("bar: exn = {!r}".format(exn))
    if bad:
        LOG("bar: Clearing frame.")
        exn.__traceback__.tb_frame.clear()
    LOG("bar: setting ev")
    ev.set()
    await asyncio.sleep(1)
    LOG("bar: foo_task = {!r}".format(foo_task))

bad = len(sys.argv) > 1 and sys.argv[1] == "bad"
foo_task = asyncio.get_event_loop().create_task(foo())
asyncio.get_event_loop().run_until_complete(bar(bad))