How to shutdown the loop and print error if coroutine raised an exception with asyncio?

Question

Suppose I have a few coroutines running in a loop. How to make so that if some of them failed with exception the whole program would fail with this exception? Because right now asyncio doesn't even prints the error messages from coroutines unless I use logging level "DEBUG".

from asyncio import get_event_loop, sleep   async def c(sleep_time=2, fail=False):     print('c', sleep_time, fail)     if fail:         raise Exception('fail')     while True:         print('doing stuff')         await sleep(sleep_time)    loop = get_event_loop() loop.create_task(c(sleep_time=10, fail=False)) loop.create_task(c(fail=True)) loop.run_forever() 

Answer 1

A graceful way is using error handling api.

https://docs.python.org/3/library/asyncio-eventloop.html#error-handling-api

Example:

import asyncio   async def run_division(a, b):     await asyncio.sleep(2)     return a / b   def custom_exception_handler(loop, context):     # first, handle with default handler     loop.default_exception_handler(context)      exception = context.get('exception')     if isinstance(exception, ZeroDivisionError):         print(context)         loop.stop()  loop = asyncio.get_event_loop()  # Set custom handler loop.set_exception_handler(custom_exception_handler) loop.create_task(run_division(1, 0)) loop.run_forever() 

Answer 2

Here are some notes that you might want use to craft your solution:

The easiest way to retrieve a couroutine's exception (or result!) is to await for it. asyncio.gather() will create tasks from coroutines and wrap all of them in one encompassing task that will fail if one of the subtasks fails:

import asyncio  import random   async def coro(n):     print("Start", n)     await asyncio.sleep(random.uniform(0.2, 0.5))     if n % 4 == 0:         raise Exception('fail ({})'.format(n))     return "OK: {}".format(n)   async def main():     tasks = [coro(i) for i in range(10)]     await asyncio.gather(*tasks)     print("done")  loop = asyncio.get_event_loop() try:     asyncio.ensure_future(main())     loop.run_forever() finally:     loop.close() 

This however does not shutdown the loop. To stop a running loop, use loop.stop(). Use this instead:

async def main():     tasks = [coro(i) for i in range(10)]     try:         await asyncio.gather(*tasks)     except Exception as e:         loop.stop()         raise     print("done") 

Stopping the loop while some long-running coroutines are running is probably not what you want. You might want to first signal some your coroutines to shut down using an event:

import asyncio  import random   async def repeat(n):     print("start", n)     while not shutting_down.is_set():         print("repeat", n)         await asyncio.sleep(random.uniform(1, 3))     print("done", n)   async def main():     print("waiting 6 seconds..")     await asyncio.sleep(6)     print("shutting down")     shutting_down.set()  # not a coroutine!     print("waiting")     await asyncio.wait(long_running)     print("done")     loop.stop()  loop = asyncio.get_event_loop() shutting_down = asyncio.Event(loop=loop) long_running = [loop.create_task(repeat(i + 1))  for i in range(5)] try:     asyncio.ensure_future(main())     loop.run_forever() finally:     loop.close() 

If you don't want to await for your tasks, you might want to use an asyncio.Event (or asyncio.Queue) to signal a global error handler to stop the loop:

import asyncio   async def fail():     try:         print("doing stuff...")         await asyncio.sleep(0.2)         print("doing stuff...")         await asyncio.sleep(0.2)         print("doing stuff...")         raise Exception('fail')     except Exception as e:         error_event.payload = e         error_event.set()         raise  # optional   async def error_handler():     await error_event.wait()     e = error_event.payload     print("Got:", e)     raise e   loop = asyncio.get_event_loop() error_event = asyncio.Event() try:     loop.create_task(fail())     loop.run_until_complete(error_handler()) finally:     loop.close() 

(Used here with run_until_complete() for simplicity, but can be used with loop.stop() as well)