python - asyncio catch TimeoutError in Task -


i have asyncio.task need cancel after amount of time. before cancelling, task needs cleanup. according docs, should able call task.cancel or asyncio.wait_for(coroutine, delay) , intercept asyncio.timeouterror in coroutine, following example not working. i've tried intercepting other errors, , calling task.cancel instead, neither have worked. misunderstanding how cancelling task works?

@asyncio.coroutine def totimeout():   try:     = 0     while true:       print("iteration ", i, "......"); += 1       yield asyncio.sleep(1)   except asyncio.timeouterror:     print("timed out")  def main():   #... stuff   yield asyncio.wait_for(totimeout(), 10)   #... more stuff  asyncio.get_event_loop().run_until_complete(main()) asyncio.get_event_loop().run_forever()

the documentation asyncio.wait_for specifies cancel underlying task, , raise timeouterror wait_for call itself:

returns result of future or coroutine. when timeout occurs, cancels task , raises asyncio.timeouterror.

and correct task cancellation can indeed intercepted:

[task.cancel] arranges cancellederror thrown wrapped coroutine on next cycle through event loop. coroutine has chance clean or deny request using try/except/finally.

note docs specify cancellederror thrown coroutine, not timeouterror.

if make adjustment, things work way expect:

import asyncio  @asyncio.coroutine def totimeout():   try:     = 0     while true:       print("iteration ", i, "......"); += 1       yield asyncio.sleep(1)   except asyncio.cancellederror:     print("timed out")  def main():   #... stuff   yield asyncio.wait_for(totimeout(), 3)   #... more stuff  asyncio.get_event_loop().run_until_complete(main())

output:

iteration  0 ...... iteration  1 ...... iteration  2 ...... timed out traceback (most recent call last):   file "aio.py", line 18, in <module>     asyncio.get_event_loop().run_until_complete(main())   file "/usr/lib/python3.4/asyncio/base_events.py", line 316, in run_until_complete     return future.result()   file "/usr/lib/python3.4/asyncio/futures.py", line 275, in result     raise self._exception   file "/usr/lib/python3.4/asyncio/tasks.py", line 238, in _step     result = next(coro)   file "aio.py", line 15, in main     yield asyncio.wait_for(totimeout(), 3)   file "/usr/lib/python3.4/asyncio/tasks.py", line 381, in wait_for     raise futures.timeouterror() concurrent.futures._base.timeouterror

as can see, 'timed out' gets printed before timeouterror raised wait_for.


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