When calling asyncio.ensure_future() on a coroutine, and if the loop is closed, ensure_future() will raise a RuntimeError. 

However, it still creates a Task, which will generate a RuntimeWarning that we can’t fix since there is no way to cancel the Task.

Here is the code to reproduce the bug:


import asyncio

l = asyncio.get_event_loop()
l.close()

async def foo():
    pass

try:
    # Since the exception raises here, fut is never set
    # so we can't call fut.cancel()
    fut = asyncio.ensure_future(foo())
except RuntimeError:
    pass


# stderr:
# aio.py:12: RuntimeWarning: coroutine 'foo' was never awaited
#   pass

Sounds like an easy fix. Could you submit a patch?

Reproduced on 3.11.

So I just realized that the OP's description is slightly misleading. (Their code is spot on though!)

The code does not create an unwaited-for *task*, assuming that "task" refers to the asyncio.Task class.

What is created is a *coroutine* object that's never awaited (as the quoted RuntimeWarning message says: "coroutine 'foo' was never awaited").

So the thing that needs to be closed in the bowels of _ensure_future() is indeed the argument (coro_or_future), in case it is a coroutine object.

New changeset 24cc6411adbfe5555ecd8901f1ea50caa414c908 by Kumar Aditya in branch 'main':
bpo-26552: Fixed case where failing `asyncio.ensure_future` did not close the coroutine (#30288)
https://github.com/python/cpython/commit/24cc6411adbfe5555ecd8901f1ea50caa414c908

Thanks Kumar for the fix!

New changeset a5451c96a14ac0c3ee7cef7b5c52ab1d783ec613 by Kumar Aditya in branch '3.10':
bpo-26552: Fixed case where failing `asyncio.ensure_future` did not close the coroutine (#30288) (#31003)
https://github.com/python/cpython/commit/a5451c96a14ac0c3ee7cef7b5c52ab1d783ec613