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GeezerVison is showing github.com — gh-157407: Shrink _PyArg_Parser and don't write builtin parsers at run time by eendebakpt · Pull Request #157531 · python/cpython

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@eendebakpt

@eendebakpt eendebakpt commented Sep 14, 2026 •
edited by bedevere-app Bot
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Make the argument clinic generated code more efficient:

  1. Store the number of positional-only parameters statically. Argument Clinic now emits .pos = N in the static _PyArg_Parser with allows _PyArg_UnpackKeywords() to skip parser_init().
  2. Reduce the size of _PyArg_Parser. pos/min/max become uint16_t and is_kwtuple_owned becomes uint8_t. custom_msg is removed: it is only needed when reporting a conversion error, so it is now looked up in the format string (";message") on that path. No in-tree parser uses ";message". _parser_init() raises SystemError for more than 65535 keywords. The struct goes from 73 bytes to 48 bytes.

Binary size change: .data / file size, main vs. this PR (Release builds):

.data .text file
Windows python316.dll (MSVC) −11,528 +48 −11,264
Windows, all *.pyd + dll −22,008 +48 −24,064
Linux python (gcc 15, static libpython) −9,120 +192
Linux, python + all *.so −17,408 +192

Possible follow-ups

With some more refactoring this could go further:

  • Non-format parsers (the common case, ~750 of ~760 in CPython) never use format, min or max. Builtin-module parsers also never use once, is_kwtuple_owned or next. A separate, smaller struct for them would save more memory, at the cost of changing the private API.
  • More static initialization: Argument Clinic could also emit fname, min and max for format-string parsers, so that builtin ones skip parser_init() too. parser_clear() then only needs to reset the kwtuple and once, since the other fields only depend on the static keywords and format.

Generated with help from Claude Code

  • Issue: Reduce some code sizes #157407

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eendebakpt and others added 2 commits September 14, 2026 22:26
@eendebakpt @claude
pythongh-136681: Store the number of positional-only parameters stati…
5169a7a 
…cally

Argument Clinic now sets _PyArg_Parser.pos statically.  Parsers with a
statically allocated keyword tuple (builtin modules) therefore need no
initialization: _PyArg_UnpackKeywords() skips parser_init() for them,
so they are never written to at run time.

A lazily built keyword tuple is now published last, with
_Py_atomic_store_ptr_release().  parser_clear() keeps pos, since it
only depends on the keywords.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>
@eendebakpt @claude
pythongh-136681: Reduce the size of _PyArg_Parser
9a7c32b 
Narrow pos, min, max and is_kwtuple_owned, which are small counts, and
remove custom_msg: it is only needed to report a conversion error, and
can then be looked up in the format string.  Keep format, keywords and
fname first, so that existing positional initializers still work.
_PyArg_Parser shrinks from 72 to 48 bytes on 64-bit platforms.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>
@eendebakpt
eendebakpt requested review from AA-Turner, ericsnowcurrently and erlend-aasland as code owners September 14, 2026 21:57
@bedevere-app bedevere-app Bot mentioned this pull request Sep 14, 2026
Reduce some code sizes #157407
Open
@bedevere-app bedevere-app Bot added the awaiting core review label Sep 14, 2026
maurycy
maurycy reviewed Sep 15, 2026
View reviewed changes
Comment thread Misc/NEWS.d/next/Core_and_Builtins/2026-09-14-12-00-00.gh-issue-157407.pQ7sLk.rst Resolved

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eendebakpt and others added 3 commits September 15, 2026 14:11
@eendebakpt @claude
pythongh-157407: Rename NEWS entry to the correct issue number
28fb676 
Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>
@eendebakpt
Merge remote-tracking branch 'origin/main' into feature/clinic-parser…
e356a09 
…-shrink

# Conflicts:
#	Modules/_ctypes/clinic/_ctypes.c.h
#	Modules/_io/clinic/bufferedio.c.h
#	Modules/_io/clinic/textio.c.h
#	Modules/_zstd/clinic/zstddict.c.h
#	Modules/clinic/_asynciomodule.c.h
#	Modules/clinic/_cursesmodule.c.h
#	Python/clinic/bltinmodule.c.h
@eendebakpt
Merge remote-tracking branch 'origin/main' into feature/clinic-parser…
a0d41bb 
…-shrink

# Conflicts:
#	Modules/_io/clinic/bufferedio.c.h
#	Modules/_io/clinic/fileio.c.h
#	Modules/_io/clinic/iobase.c.h
#	Modules/_io/clinic/textio.c.h
#	Modules/_io/clinic/winconsoleio.c.h
@eendebakpt

eendebakpt commented Sep 26, 2026 •
edited
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Update: the followup ideas have been implemented in main...eendebakpt:feature/clinic-kwparser. That results in

.text .rdata .data file
python316.dll +1,152 −624 −28,744 −31,232
all *.pyd + dll +1,152 −2,112 −44,440 −49,664

I will look into opening a PR for that as an alternative for this PR.

Update: and some more reduction: eendebakpt/cpython@main...feature/clinic-kwparser-split
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@ericsnowcurrently ericsnowcurrently Awaiting requested review from ericsnowcurrently ericsnowcurrently is a code owner
@erlend-aasland erlend-aasland Awaiting requested review from erlend-aasland erlend-aasland is a code owner
@AA-Turner AA-Turner Awaiting requested review from AA-Turner AA-Turner is a code owner
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@maurycy maurycy maurycy left review comments
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@eendebakpt @maurycy
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