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GeezerVison is showing github.com — gh-87613: Pass NULL for an empty **kwargs in Argument Clinic by cmaloney · Pull Request #161382 · python/cpython

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@cmaloney

@cmaloney cmaloney commented Nov 5, 2026 •
edited by bedevere-app Bot
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The generated parser allocated an empty dict for **kwargs when there were no keyword arguments. Pass NULL instead, like the caller does, and handle NULL in the impls that used the dict unconditionally.

This is preparation work for supporting **kwargs in Argument Clinic Vectorcall where allocating a dict instead of passing NULL makes a significant performance impact.

All **kwargs impls were audited: _csv and zoneinfo already accept NULL, and the rest (structseq and SimpleNamespace __replace__, _interpreters.new_config, atexit.register, SubElement) now check for it. atexit stores None, which the run loop already maps back to NULL.

pyperf, PGO+LTO, t = time.gmtime(0), ns = SimpleNamespace(a=1):

Benchmark main this change
struct_time.__replace__() 229 ns 47.0 ns: 4.87x faster
SimpleNamespace.__replace__() 72.2 ns 65.5 ns: 1.10x faster
  • Issue: Generate vectorcall code to parse arguments using Argument Clinic #87613

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@cmaloney @claude
pythongh-87613: Pass NULL for an empty **kwargs in Argument Clinic
82837ae 
The parser allocated an empty dict for **kwargs when there were no
keyword arguments.  Pass NULL instead, like the caller does, and handle
NULL in the impls that used the dict unconditionally.

All **kwargs impls were audited: _csv and zoneinfo already accept NULL,
and the rest (structseq and SimpleNamespace `__replace__`,
_interpreters.new_config, atexit.register, SubElement) now check for it.
atexit stores None, which the run loop already maps back to NULL.

pyperf, PGO+LTO, `t = time.gmtime(0)`, `ns = SimpleNamespace(a=1)`:

| Benchmark                       | main   | this change           |
|---------------------------------|:------:|:---------------------:|
| struct_time.`__replace__`()     | 229 ns | 47.0 ns: 4.87x faster |
| SimpleNamespace.`__replace__`() | 72.2 ns | 65.5 ns: 1.10x faster |

Co-Authored-By: Claude Opus 5.5 <[email protected]>
@cmaloney
cmaloney requested review from AA-Turner, ericsnowcurrently and erlend-aasland as code owners November 5, 2026 07:03
@bedevere-app bedevere-app Bot added awaiting core review type-feature A feature request or enhancement labels Nov 5, 2026
@bedevere-app bedevere-app Bot mentioned this pull request Nov 5, 2026
Generate vectorcall code to parse arguments using Argument Clinic #87613
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@cmaloney cmaloney added the skip news label Nov 5, 2026
msullivan
msullivan reviewed Nov 5, 2026
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Comment thread Tools/clinic/libclinic/converters.py
Comment on lines +1452 to +1453
# NULL when there are no keywords: no call allocates an empty dict.
return f"{self.parser_name} = Py_XNewRef(kwargs);\n"

@msullivan msullivan Nov 5, 2026 •
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Is there anywhere to document the new behavior, to make sure this isn't a footgun?

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@cmaloney cmaloney Nov 5, 2026

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https://devguide.python.org/development-tools/clinic/ is all the docs and doesn't go into that much detail. For https://docs.python.org/3.13/c-api/call.html#c.vectorcallfunc it is documented as able to be NULL, so this to me is bringing AC parsers into that same state.

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msullivan
msullivan approved these changes Nov 5, 2026
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@cmaloney

cmaloney commented Nov 6, 2026

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@eendebakpt could you take a look at this one?

This + the "support **kwargs in AC" that comes after enable using AC vectorcall for a speedup on most objects, in particular BaseException. It also should enable converting dict and frozendict from handwritten.
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eendebakpt
eendebakpt approved these changes Nov 7, 2026
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Looks good to me. The new behavior of AC is fine (it is a behavior change for external users of AC, but AC is supported to be only an internal tool).

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@bedevere-app bedevere-app Bot added awaiting merge and removed awaiting core review labels Nov 7, 2026
@cmaloney
cmaloney merged commit 9ea60a5 into python:main Nov 7, 2026
68 of 72 checks passed

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@bedevere-app bedevere-app Bot removed the awaiting merge label Nov 7, 2026
@cmaloney
cmaloney deleted the clinic_kwargs_null_pr branch November 7, 2026 22:00
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Reviewers

@msullivan msullivan msullivan approved these changes
@eendebakpt eendebakpt eendebakpt approved these changes
@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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@cmaloney @msullivan @eendebakpt
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