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GeezerVison is showing github.com — gh-159964: Propagate contextvars to ThreadPoolExecutor tasks by methatp0 · Pull Request #160024 · python/cpython

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@methatp0

@methatp0 methatp0 commented Oct 21, 2026 •
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Fixes gh-159964.

Capture a copy of the submitting thread's context for each submitted task and execute the callable within that context. This prevents a task from inheriting stale context from whichever worker happened to start first and keeps context-variable rebinding in one task isolated from subsequent tasks.

The change includes a regression test for context propagation across reused workers, documentation, and a NEWS entry.

Validation: lint and the C API documentation check pass. The full CI matrix is running; the local CPython test suite could not be run in this environment.

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@python-cla-bot

python-cla-bot Bot commented Oct 21, 2026

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The following commit authors need to sign the Contributor License Agreement:

  • [email protected]

CLA not signed
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@bedevere-app

bedevere-app Bot commented Oct 21, 2026

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Most changes to Python require a NEWS entry. Add one using the blurb_it web app or the blurb command-line tool.

If this change has little impact on Python users, wait for a maintainer to apply the skip news label instead.
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@methatp0 methatp0 changed the title Fix ThreadPoolExecutor context propagation gh-159964: Propagate contextvars to ThreadPoolExecutor tasks Oct 21, 2026
@bedevere-app bedevere-app Bot mentioned this pull request Oct 21, 2026
ThreadPoolExecutor tasks see the context of whichever submit() call started their worker thread #159964
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@methatp0
pythongh-159964: Propagate contextvars to ThreadPoolExecutor tasks
526dfe8
@methatp0
methatp0 force-pushed the fix-threadpool-contextvars-submit branch from 8731f40 to 526dfe8 Compare October 21, 2026 05:25
@methatp0
methatp0 marked this pull request as ready for review October 21, 2026 05:25
@bedevere-app bedevere-app Bot added the awaiting review label Oct 21, 2026
@read-the-docs-community

read-the-docs-community Bot commented Oct 21, 2026

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Documentation build overview

📚 cpython-previews | 🛠️ Build #35288083 | 📁 Comparing 526dfe8 against main (b577a54)

  🔍 Preview build  

2 files changed
± library/concurrent.futures.html
± whatsnew/changelog.html
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@ngoldbaum

ngoldbaum commented Oct 21, 2026

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Thanks for the patch. I was planning to work on this issue, and I’ll take it from here. The proposed change fixes the reproducer, but unconditional context propagation also changes downstream behavior. I want to think about compatibility questions and performance before settling on an implementation. I’m closing this PR to avoid parallel work; I’ll follow up on the issue.
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@ngoldbaum ngoldbaum closed this Oct 21, 2026
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ThreadPoolExecutor tasks see the context of whichever submit() call started their worker thread

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@methatp0 @ngoldbaum
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