gh-143054: Disallow non-top-level Cut for now #143622
Merged
+85
−3
Conversation
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
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.
encukou
requested review from
AA-Turner,
lysnikolaou,
pablogsal and
willingc
as code owners
pablogsal
approved these changes
Jan 9, 2026
Member
pablogsal
left a comment
pablogsal
left a comment
There was a problem hiding this comment.
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
approved these changes
Jan 12, 2026
Member
lysnikolaou
left a comment
lysnikolaou
left a comment
There was a problem hiding this comment.
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.
Member
Author
|
Thank you for the reviews -- and for pegen itself! |
encukou
added
needs backport to 3.14
|
Thanks @encukou for the PR 🌮🎉.. I'm working now to backport this PR to: 3.14. |
miss-islington
pushed a commit
to miss-islington/cpython
that referenced
this pull request
Jan 13, 2026
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. |
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
3 participants
Add this suggestion to a batch that can be applied as a single commit.
This suggestion is invalid because no changes were made to the code.
Suggestions cannot be applied while the pull request is closed.
Suggestions cannot be applied while viewing a subset of changes.
Only one suggestion per line can be applied in a batch.
Add this suggestion to a batch that can be applied as a single commit.
Applying suggestions on deleted lines is not supported.
You must change the existing code in this line in order to create a valid suggestion.
Outdated suggestions cannot be applied.
This suggestion has been applied or marked resolved.
Suggestions cannot be applied from pending reviews.
Suggestions cannot be applied on multi-line comments.
Suggestions cannot be applied while the pull request is queued to merge.
Suggestion cannot be applied right now. Please check back later.
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/