[Python-checkins] bpo-34790: [docs] Passing coroutines to asyncio.wait() can be confusing. (GH-9543)
Miss Islington (bot)
webhook-mailer at python.org
Tue Sep 25 14:57:54 EDT 2018
https://github.com/python/cpython/commit/3cc9557d9f03086fa3b0ea166f8506087e6950e3
commit: 3cc9557d9f03086fa3b0ea166f8506087e6950e3
branch: 3.7
author: Miss Islington (bot) <31488909+miss-islington at users.noreply.github.com>
committer: GitHub <noreply at github.com>
date: 2018-09-25T11:57:49-07:00
summary:
bpo-34790: [docs] Passing coroutines to asyncio.wait() can be confusing. (GH-9543)
(cherry picked from commit 996859a90df51f84eab47351702cb59c6db4428a)
Co-authored-by: Yury Selivanov <yury at magic.io>
files:
A Misc/NEWS.d/next/Documentation/2018-09-24-12-47-08.bpo-34790.G2KXIH.rst
M Doc/library/asyncio-task.rst
diff --git a/Doc/library/asyncio-task.rst b/Doc/library/asyncio-task.rst
index bdb475a797db..7fbfd426dba6 100644
--- a/Doc/library/asyncio-task.rst
+++ b/Doc/library/asyncio-task.rst
@@ -466,14 +466,20 @@ Waiting Primitives
return_when=ALL_COMPLETED)
Run :ref:`awaitable objects <asyncio-awaitables>` in the *aws*
- sequence concurrently and block until the condition specified
+ set concurrently and block until the condition specified
by *return_when*.
If any awaitable in *aws* is a coroutine, it is automatically
- scheduled as a Task.
+ scheduled as a Task. Passing coroutines objects to
+ ``wait()`` directly is deprecated as it leads to
+ :ref:`confusing behavior <asyncio_example_wait_coroutine>`.
Returns two sets of Tasks/Futures: ``(done, pending)``.
+ Usage::
+
+ done, pending = await asyncio.wait(aws)
+
The *loop* argument is deprecated and scheduled for removal
in Python 4.0.
@@ -508,9 +514,35 @@ Waiting Primitives
Unlike :func:`~asyncio.wait_for`, ``wait()`` does not cancel the
futures when a timeout occurs.
- Usage::
+ .. _asyncio_example_wait_coroutine:
+ .. note::
- done, pending = await asyncio.wait(aws)
+ ``wait()`` schedules coroutines as Tasks automatically and later
+ returns those implicitly created Task objects in ``(done, pending)``
+ sets. Therefore the following code won't work as expected::
+
+ async def foo():
+ return 42
+
+ coro = foo()
+ done, pending = await asyncio.wait({coro})
+
+ if coro in done:
+ # This branch will never be run!
+
+ Here is how the above snippet can be fixed::
+
+ async def foo():
+ return 42
+
+ task = asyncio.create_task(foo())
+ done, pending = await asyncio.wait({task})
+
+ if task in done:
+ # Everything will work as expected now.
+
+ Passing coroutine objects to ``wait()`` directly is
+ deprecated.
.. function:: as_completed(aws, \*, loop=None, timeout=None)
diff --git a/Misc/NEWS.d/next/Documentation/2018-09-24-12-47-08.bpo-34790.G2KXIH.rst b/Misc/NEWS.d/next/Documentation/2018-09-24-12-47-08.bpo-34790.G2KXIH.rst
new file mode 100644
index 000000000000..dc3de2c7d4c8
--- /dev/null
+++ b/Misc/NEWS.d/next/Documentation/2018-09-24-12-47-08.bpo-34790.G2KXIH.rst
@@ -0,0 +1 @@
+Document how passing coroutines to asyncio.wait() can be confusing.
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