[Python-checkins] bpo-32309: Add support for contextvars in asyncio.to_thread() (GH-20278)
Kyle Stanley
webhook-mailer at python.org
Thu May 21 01:20:50 EDT 2020
https://github.com/python/cpython/commit/0f56263e62ba91d0baae40fb98947a3a98034a73
commit: 0f56263e62ba91d0baae40fb98947a3a98034a73
branch: master
author: Kyle Stanley <aeros167 at gmail.com>
committer: GitHub <noreply at github.com>
date: 2020-05-20T22:20:43-07:00
summary:
bpo-32309: Add support for contextvars in asyncio.to_thread() (GH-20278)
Allows contextvars from the main thread to be accessed in the separate thread used in `asyncio.to_thread()`. See the [discussion](https://github.com/python/cpython/pull/20143#discussion_r427808225) in GH-20143 for context.
Automerge-Triggered-By: @aeros
files:
M Doc/library/asyncio-task.rst
M Lib/asyncio/threads.py
M Lib/test/test_asyncio/test_threads.py
diff --git a/Doc/library/asyncio-task.rst b/Doc/library/asyncio-task.rst
index 7c2704090551b..dd94c14854835 100644
--- a/Doc/library/asyncio-task.rst
+++ b/Doc/library/asyncio-task.rst
@@ -610,7 +610,9 @@ Running in Threads
Asynchronously run function *func* in a separate thread.
Any \*args and \*\*kwargs supplied for this function are directly passed
- to *func*.
+ to *func*. Also, the current :class:`contextvars.Context` is propogated,
+ allowing context variables from the event loop thread to be accessed in the
+ separate thread.
Return an :class:`asyncio.Future` which represents the eventual result of
*func*.
@@ -657,6 +659,8 @@ Running in Threads
that release the GIL or alternative Python implementations that don't
have one, `asyncio.to_thread()` can also be used for CPU-bound functions.
+ .. versionadded:: 3.9
+
Scheduling From Other Threads
=============================
diff --git a/Lib/asyncio/threads.py b/Lib/asyncio/threads.py
index 2f40467fe5bc7..51e0ba95d822e 100644
--- a/Lib/asyncio/threads.py
+++ b/Lib/asyncio/threads.py
@@ -1,6 +1,7 @@
"""High-level support for working with threads in asyncio"""
import functools
+import contextvars
from . import events
@@ -12,10 +13,13 @@ async def to_thread(func, /, *args, **kwargs):
"""Asynchronously run function *func* in a separate thread.
Any *args and **kwargs supplied for this function are directly passed
- to *func*.
+ to *func*. Also, the current :class:`contextvars.Context` is propogated,
+ allowing context variables from the main thread to be accessed in the
+ separate thread.
Return an asyncio.Future which represents the eventual result of *func*.
"""
loop = events.get_running_loop()
- func_call = functools.partial(func, *args, **kwargs)
+ ctx = contextvars.copy_context()
+ func_call = functools.partial(ctx.run, func, *args, **kwargs)
return await loop.run_in_executor(None, func_call)
diff --git a/Lib/test/test_asyncio/test_threads.py b/Lib/test/test_asyncio/test_threads.py
index 99a00f21832f3..2af322421dacf 100644
--- a/Lib/test/test_asyncio/test_threads.py
+++ b/Lib/test/test_asyncio/test_threads.py
@@ -3,6 +3,7 @@
import asyncio
import unittest
+from contextvars import ContextVar
from unittest import mock
from test.test_asyncio import utils as test_utils
@@ -74,6 +75,19 @@ async def main():
self.loop.run_until_complete(main())
func.assert_called_once_with('test', something=True)
+ def test_to_thread_contextvars(self):
+ test_ctx = ContextVar('test_ctx')
+
+ def get_ctx():
+ return test_ctx.get()
+
+ async def main():
+ test_ctx.set('parrot')
+ return await asyncio.to_thread(get_ctx)
+
+ result = self.loop.run_until_complete(main())
+ self.assertEqual(result, 'parrot')
+
if __name__ == "__main__":
unittest.main()
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