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gh-136157: Optimize asyncio.to_thread to avoid contextvars.copy_context() overhead for empty contexts #136159

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gh-136157: Optimize asyncio.to_thread to avoid contextvars.copy_context() overhead for empty contexts #136159

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df9f12f
Optimize asyncio.to_thread to avoid contextvars.copy_context() overhe…
heliang666s Jul 1, 2025
5c758d5
Fix DeprecationWarning in asyncio tests
heliang666s Jul 1, 2025
9ed2acb
📜🤖 Added by blurb_it.
blurb-it[bot] Jul 1, 2025
56035d4
Use private _set_event_loop_policy API to avoid DeprecationWarning
heliang666s Jul 1, 2025
870f818
use bool to replace len()
heliang666s Jul 1, 2025
28622dc
Update Misc/NEWS.d/next/Library/2025-07-01-02-19-00.gh-issue-136157.j…
heliang666s Jul 1, 2025
5d0582d
Update 2025-07-01-02-19-00.gh-issue-136157.jGs_zV.rst
heliang666s Jul 1, 2025
ac322ea
Update 2025-07-01-02-19-00.gh-issue-136157.jGs_zV.rst
heliang666s Jul 1, 2025
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7 changes: 5 additions & 2 deletions Lib/asyncio/threads.py
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Original file line number Diff line number Diff line change
Expand Up @@ -21,5 +21,8 @@ async def to_thread(func, /, *args, **kwargs):
"""
loop = events.get_running_loop()
ctx = contextvars.copy_context()
func_call = functools.partial(ctx.run, func, *args, **kwargs)
return await loop.run_in_executor(None, func_call)
if not ctx:
callback = functools.partial(func, *args, **kwargs)
else:
callback = functools.partial(ctx.run, func, *args, **kwargs)
Comment on lines +24 to +27
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Hm, couldn't we alternatively add a fast path to Context.run?

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I don't think so. Please see the issue for details.

return await loop.run_in_executor(None, callback)
36 changes: 36 additions & 0 deletions Lib/test/test_asyncio/test_threads.py
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Expand Up @@ -2,6 +2,7 @@

import asyncio
import unittest
import functools

from contextvars import ContextVar
from unittest import mock
Expand Down Expand Up @@ -61,6 +62,41 @@ def get_ctx():

self.assertEqual(result, 'parrot')

@mock.patch('asyncio.base_events.BaseEventLoop.run_in_executor')
async def test_to_thread_optimization_path(self, run_in_executor):
# This test ensures that `to_thread` uses the correct execution path
# based on whether the context is empty or not.

# `to_thread` awaits the future returned by `run_in_executor`.
# We need to provide a completed future as a return value for the mock.
fut = asyncio.Future()
fut.set_result(None)
run_in_executor.return_value = fut

def myfunc():
pass

# Test with an empty context (optimized path)
await asyncio.to_thread(myfunc)
run_in_executor.assert_called_once()

callback = run_in_executor.call_args.args[1]
self.assertIsInstance(callback, functools.partial)
self.assertIs(callback.func, myfunc)
run_in_executor.reset_mock()

# Test with a non-empty context (standard path)
var = ContextVar('var')
var.set('value')

await asyncio.to_thread(myfunc)
run_in_executor.assert_called_once()

callback = run_in_executor.call_args.args[1]
self.assertIsInstance(callback, functools.partial)
self.assertIsNot(callback.func, myfunc) # Should be ctx.run
self.assertIs(callback.args[0], myfunc)


if __name__ == "__main__":
unittest.main()
1 change: 1 addition & 0 deletions Misc/NEWS.d/next/Library/2025-07-01-02-19-00.gh-issue-136157.jGs_zV.rst
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@@ -0,0 +1 @@
Optimized :func:`asyncio.to_thread` to avoid unnecessary performance overhead from calling :meth:`contextvars.Context.run` when the context is empty.
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