[3.14] gh-143054: Disallow non-top-level Cut for now (GH-143622) #143790

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miss-islington wants to merge 1 commit into python:3.14

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[3.14] gh-143054: Disallow non-top-level Cut for now (GH-143622) #143790

miss-islington wants to merge 1 commit into python:3.14 from miss-islington:backport-f0a0467-3.14

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@miss-islington miss-islington commented Jan 13, 2026 •

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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.
(cherry picked from commit f0a0467)

Co-authored-by: Petr Viktorin encukou@gmail.com

Issue: Cut (~) operator in grammar unclear #143054

📚 Documentation preview 📚: https://cpython-previews--143790.org.readthedocs.build/

@encukou @miss-islington


 pythongh-143054 : Disallow non-top-level Cut for now ( pythonGH-143622 )

e7c4996

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>