Test gather cancellation and resulting state.

gather_cancellation_test.py

# -*- coding: utf-8 -*-
"""Test gather cancellation and resulting state.

SeeAlso:
    https://stackoverflow.com/questions/61942306/asyncio-gather-task-cancelled-is-false-after-task-cancel

Here's my take on what's happening:

    * future_gather.cancel() cancels task_child
    * await future_gather will cause task_child to run and raise CancelledError
    * task_child's done callback for future_gather is invoked
    * _done_callback(task_child) calls future_gather.set_exception(CancelledError()) which sets:
        * future_gather._exception = CancelledError()
        * future_gather._state = _FINISHED
    * await calls future_gather.result() which raises self._exception if set

Therefore the end result is a future whose:
    * .cancel() has been called
    * has explicitly raised a CancelledError to clients
    * returns a CancelledError for .exception()
    * returns False for .cancelled()
    * has a _state of FINISHED (not CANCELLED)
"""

import logging
from asyncio import CancelledError
from asyncio import ensure_future
from asyncio import gather
from asyncio import run
from asyncio import sleep


logging.basicConfig(level=logging.INFO)
logger = logging.getLogger(__name__)


async def main():
    """Cancel a gather() future and child and return it."""

    task_child = ensure_future(sleep(1.0))
    future_gather = gather(task_child)

    future_gather.cancel()
    try:
        await future_gather
    except CancelledError:
        pass

    return future_gather, task_child


# run
future_gather, task_child = run(main())

# log gather state
logger.info(f"gather:")
logger.info(f"cancelled: {future_gather.cancelled()}")
logger.info(f"done: {future_gather.done()}")
logger.info(f"exception: {future_gather.exception()!r}")
logger.info(f"state: {future_gather._state}")

logger.info(f"")

# log child state
logger.info(f"child:")
logger.info(f"cancelled: {task_child.cancelled()}")
logger.info(f"done: {task_child.done()}")
# logger.info(f"exception: {task_child.exception()}")  Raises because _state is CANCELLED
logger.info(f"state: {task_child._state}")