[Python-checkins] cpython (3.4): asyncio doc: add a section about task cancellation
victor.stinner
python-checkins at python.org
Fri Jan 30 00:56:20 CET 2015
https://hg.python.org/cpython/rev/173d4a8e1e74
changeset: 94387:173d4a8e1e74
branch: 3.4
parent: 94385:21010940f8c1
user: Victor Stinner <victor.stinner at gmail.com>
date: Fri Jan 30 00:55:58 2015 +0100
summary:
asyncio doc: add a section about task cancellation
files:
Doc/library/asyncio-dev.rst | 37 +++++++++++++++++++++++++
1 files changed, 37 insertions(+), 0 deletions(-)
diff --git a/Doc/library/asyncio-dev.rst b/Doc/library/asyncio-dev.rst
--- a/Doc/library/asyncio-dev.rst
+++ b/Doc/library/asyncio-dev.rst
@@ -40,6 +40,43 @@
<asyncio-logger>`.
+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::
+
+ if not fut.cancelled():
+ fut.set_result('done')
+
+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::
+
+ @coroutine
+ def slow_operation(fut):
+ if fut.cancelled():
+ return
+ # ... slow computation ...
+ yield from fut
+ # ...
+
+The :func:`shield` function can also be used to ignore cancellation.
+
+
.. _asyncio-multithreading:
Concurrency and multithreading
--
Repository URL: https://hg.python.org/cpython
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