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

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 @@ class ToThreadTests(test_utils.TestCase):
         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()