Function shield

Lib/asyncio/tasks.py:930–1004

Wait for a future, shielding it from cancellation. The statement task = asyncio.create_task(something()) res = await shield(task) is exactly equivalent to the statement res = await something() *except* that if the coroutine containing it is cancelled, the

(arg)

def shield(arg):
    """Wait for a future, shielding it from cancellation.

    The statement

        task = asyncio.create_task(something())
        res = await shield(task)

    is exactly equivalent to the statement

        res = await something()

    *except* that if the coroutine containing it is cancelled, the
    task running in something() is not cancelled.  From the POV of
    something(), the cancellation did not happen.  But its caller is
    still cancelled, so the yield-from expression still raises
    CancelledError.  Note: If something() is cancelled by other means
    this will still cancel shield().

    If you want to completely ignore cancellation (not recommended)
    you can combine shield() with a try/except clause, as follows:

        task = asyncio.create_task(something())
        try:
            res = await shield(task)
        except CancelledError:
            res = None

    Save a reference to tasks passed to this function, to avoid
    a task disappearing mid-execution. The event loop only keeps
    weak references to tasks. A task that isn't referenced elsewhere
    may get garbage collected at any time, even before it's done.
    """
    inner = ensure_future(arg)
    if inner.done():
        # Shortcut.
        return inner
    loop = futures._get_loop(inner)
    outer = loop.create_future()

    if loop is not None and (cur_task := current_task(loop)) is not None:
        futures.future_add_to_awaited_by(inner, cur_task)
    else:
        cur_task = None

    def _clear_awaited_by_callback(inner):
        futures.future_discard_from_awaited_by(inner, cur_task)

    def _inner_done_callback(inner):
        if outer.cancelled():
            return

        if inner.cancelled():
            outer.cancel()
        else:
            exc = inner.exception()
            if exc is not None:
                outer.set_exception(exc)