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Title: gh-149557: Make itertools.pairwise safe under re-entry and concurrency by eendebakpt · Pull Request #150589 · python/cpython · GitHub

Open Graph Title: gh-149557: Make itertools.pairwise safe under re-entry and concurrency by eendebakpt · Pull Request #150589 · python/cpython

X Title: gh-149557: Make itertools.pairwise safe under re-entry and concurrency by eendebakpt · Pull Request #150589 · python/cpython

Description: Add a re-entrancy guard so a same-thread re-entrant call (e.g. when the input iterator's __next__ calls back into pairwise.__next__) raises RuntimeError instead of crashing or producing garbled output. Matches the behavior of itertools.tee and of generators, and fixes the use-after-free crash reported in gh-149557. (instead of raising RuntimeError we could consider raising ValueError) Wrap pairwise.__next__ in a critical section so concurrent callers from different threads serialize cleanly in the free-threaded build. Also addresses #123471

Open Graph Description: Add a re-entrancy guard so a same-thread re-entrant call (e.g. when the input iterator's __next__ calls back into pairwise.__next__) raises RuntimeError instead of crashing or producing garble...

X Description: Add a re-entrancy guard so a same-thread re-entrant call (e.g. when the input iterator's __next__ calls back into pairwise.__next__) raises RuntimeError instead of crashing or producing ga...

Opengraph URL: https://github.com/python/cpython/pull/150589

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103c022 gh-149557, gh-123471: Make itertools.pairwise safe under concurrency … eendebakpt May 29, 2026 https://github.com/python/cpython/pull/150589/commits/103c0220068675894d16c9fde7aceaa6452479b7
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