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/20143GH-discussion_r427808225) in GH-20143 for context. Automerge-Triggered-By: @aeros (cherry picked from commit 0f56263) Co-authored-by: Kyle Stanley <aeros167@gmail.com>

Author: miss-islington

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
 =============================

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)

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()