Cancellation

Cancellation of tasks is not common in classic programming. In asynchronous

programming, not only it is something common, but you have to prepare your

code to handle it.

Futures and tasks can be cancelled explicitly with their :meth:`Future.cancel`

method. The :func:`wait_for` function cancels the waited task when the timeout

occurs. There are many other cases where a task can be cancelled indirectly.

Don't call :meth:`~Future.set_result` or :meth:`~Future.set_exception` method

of :class:`Future` if the future is cancelled: it would fail with an exception.

For example, write::

Don't schedule directly a call to the :meth:`~Future.set_result` or the

:meth:`~Future.set_exception` method of a future with

:meth:`BaseEventLoop.call_soon`: the future can be cancelled before its method

is called.

If you wait for a future, you should check early if the future was cancelled to

avoid useless operations. Example::

The :func:`shield` function can also be used to ignore cancellation.