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gh-143054: Disallow non-top-level Cut for now #143622
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The behaviour of Cut in nested parentheses, Repeat, Opt, and similar is somewhat chaotic. Apparently even the academic papers on PEG aren't as clear as they could be. And it doesn't really matter. Python only uses top-level cuts. When that changes, we can clarify as much as necessary (and even change the implementation to make sense for what we'll need). Document that this is deliberately unspecified, and add a test to make sure any decision is deliberate, tested and documented.
@pablogsal
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This LGTM! I feel you regarding how chaotic this is and how unintuitive the consequences are so I think adding these tests and the docs is perfect and improves the situation. Thanks a lot!
I will recommend to wait for @lysnikolaou to take a look if possible
@lysnikolaou
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LGTM! This certainly is an improvement over leaving stuff undocumented. I doubt there'll be the need for non-top-level cuts anytime soon anyway.
encukou
commented
Jan 13, 2026
Thank you for the reviews -- and for pegen itself!
Thanks @encukou for the PR 🌮🎉.. I'm working now to backport this PR to: 3.14.
🐍🍒⛏🤖
The behaviour of Cut in nested parentheses, Repeat, Opt, and similar is somewhat chaotic. Apparently even the academic papers on PEG aren't as clear as they could be. And it doesn't really matter. Python only uses top-level cuts. When that changes, we can clarify as much as necessary (and even change the implementation to make sense for what we'll need). Document that this is deliberately unspecified, and add a test to make sure any decision is deliberate, tested and documented. (cherry picked from commit f0a0467) Co-authored-by: Petr Viktorin <encukou@gmail.com>
GH-143790 is a backport of this pull request to the 3.14 branch.
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The behaviour of Cut in nested parentheses, Repeat, Opt, and similar is somewhat chaotic. Apparently even the academic papers on PEG aren't as clear as they could be.
And it doesn't really matter. Python only uses top-level cuts. When/if that changes, we can clarify as much as necessary (and even change the implementation to make sense for what we'll need).
For now, document that this is deliberately unspecified, and add a test to make sure any decision is deliberate, tested and documented.
📚 Documentation preview 📚: https://cpython-previews--143622.org.readthedocs.build/