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GeezerVison is showing github.com — gh-153568: Stop re-walking memo lists when growing left-recursive rules by pablogsal · Pull Request #153574 · python/cpython

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@pablogsal

@pablogsal pablogsal commented Jul 11, 2026 •
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The grow-the-seed loop of every left-recursive rule searched the token's memo list on each iteration to update its own entry, even though it inserted that entry itself on the first pass. Keeping a direct reference and writing through it removes the repeated walks.

Benchmark (parsing 8 of the largest stdlib files, 1.3 MB, 20 times per run with _PyParser_ASTFromString — parser only, no AST-to-Python conversion; pyperf, interleaved runs):

build time per run speedup
main 1.74 s ± 0.03
this PR 1.73 s ± 0.03 within noise** (read below)

On its own the effect is below measurement noise; combined with the memo filter of #153571 it measured about 3% because the remaining walks there are the long ones this change removes.

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@pablogsal
pythongh-153568: Stop re-walking memo lists when growing left-recursi…
678e3a3 
…ve rules

The left-recursion driver now keeps a direct reference to its memo
entry and updates it in place on every iteration.
@pablogsal
pablogsal requested a review from lysnikolaou as a code owner July 11, 2026 13:59
@bedevere-app bedevere-app Bot mentioned this pull request Jul 11, 2026
Speed up the parser #153568
Open
@bedevere-app bedevere-app Bot added the awaiting core review label Jul 11, 2026
@pablogsal pablogsal mentioned this pull request Sep 13, 2026
gh-153568: Parse arithmetic with operator loops #157413
Open
@pablogsal
pablogsal merged commit ac86e12 into python:main Sep 14, 2026
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@pablogsal
pablogsal deleted the gh-153568-leftrec-memo branch September 14, 2026 15:41
@bedevere-app bedevere-app Bot removed the awaiting core review label Sep 14, 2026
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@lysnikolaou lysnikolaou Awaiting requested review from lysnikolaou lysnikolaou is a code owner

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@pablogsal
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