How to await Tasks created inside an endless loop in asyncio - python-asyncio

Question

I want to create tasks programatically inside the running loop. Every second will be check if some tasks are currently not running (sometimes because of exceptions) the should be started. For simplicity reasons here ist only the loop.
import asyncio
async def parent_coro():
await child_coro()
async def child_coro():
await asyncio.sleep(1)
raise Exception('OMG!')
async def main():
tasks = []
while True:
task = asyncio.create_task(parent_coro()),
tasks.append(task)
await asyncio.sleep(1)
# This cannot be reached
done, pending = await asyncio.wait(tasks)
for task in done:
try:
print(task.exception())
except Exception:
print('You will not see me!')
if __name__ == '__main__':
asyncio.run(main())
The problem is that this Tasks cannot be awaited, and when I stop the process (Ctrl+C) I get Task exception was never retrieved. Here the stacktrace:
^CTraceback (most recent call last):
File "/tmp/check.py", line 33, in <module>
asyncio.run(main())
File "/home/antonio/.pyenv/versions/3.9.6/lib/python3.9/asyncio/runners.py", line 44, in run
return loop.run_until_complete(main)
File "/home/antonio/.pyenv/versions/3.9.6/lib/python3.9/asyncio/base_events.py", line 629, in run_until_complete
self.run_forever()
File "/home/antonio/.pyenv/versions/3.9.6/lib/python3.9/asyncio/base_events.py", line 596, in run_forever
self._run_once()
File "/home/antonio/.pyenv/versions/3.9.6/lib/python3.9/asyncio/base_events.py", line 1854, in _run_once
event_list = self._selector.select(timeout)
File "/home/antonio/.pyenv/versions/3.9.6/lib/python3.9/selectors.py", line 469, in select
fd_event_list = self._selector.poll(timeout, max_ev)
KeyboardInterrupt
Task exception was never retrieved
future: <Task finished name='Task-6' coro=<parent_coro() done, defined at /home/myproject/check.py:4> exception=Exception('OMG!')>
Traceback (most recent call last):
File "/tmp/check.py", line 5, in parent_coro
await child_coro()
File "/tmp/check.py", line 10, in child_coro
raise Exception('OMG!')
Exception: OMG!
Task exception was never retrieved
future: <Task finished name='Task-5' coro=<parent_coro() done, defined at /tmp/check.py:4> exception=Exception('OMG!')>
Traceback (most recent call last):
File "/tmp/check.py", line 5, in parent_coro
await child_coro()
File "/tmp/check.py", line 10, in child_coro
raise Exception('OMG!')
Exception: OMG!
Task exception was never retrieved
future: <Task finished name='Task-4' coro=<parent_coro() done, defined at /tmp/check.py:4> exception=Exception('OMG!')>
Traceback (most recent call last):
File "/tmp/check.py", line 5, in parent_coro
await child_coro()
File "/tmp/check.py", line 10, in child_coro
raise Exception('OMG!')
Exception: OMG!
Task exception was never retrieved
future: <Task finished name='Task-3' coro=<parent_coro() done, defined at /home/myproject/check.py:4> exception=Exception('OMG!')>
Traceback (most recent call last):
File "/home/myproject/check.py", line 5, in parent_coro
await child_coro()
File "/home/myproject/check.py", line 10, in child_coro
raise Exception('OMG!')
Exception: OMG!
Task exception was never retrieved
future: <Task finished name='Task-2' coro=<parent_coro() done, defined at /home/myproject/check.py:4> exception=Exception('OMG!')>
Traceback (most recent call last):
File "/home/myproject/check.py", line 5, in parent_coro
await child_coro()
File "/home/myproject/check.py", line 10, in child_coro
raise Exception('OMG!')
Exception: OMG!

Answer 1

There is now way for your code to exit the while loop.
As a solution, you can inspect your tasks inside the while loop. Your tasks will then be inspected every second.
async def main():
tasks = []
while True:
task = asyncio.create_task(parent_coro()),
tasks.append(task)
await asyncio.sleep(1)
done, pending = await asyncio.wait(tasks)
for task in done:
try:
print(task.exception())
except Exception:
print('You will not see me!)
You could also create a background task launching parent_coro tasks every second. In this case, once this task is launched, you could check tasks whenever you want.