gh-121676: Raise a ``DeprecationWarning`` if the Python implementation of ``functools.reduce`` is called with a keyword args by Eclips4 · Pull Request #121677 · python/cpython · GitHub

Issue: Python implementation of functools.reduce accepts keyword arguments, while the C implementation does not #121676

LGTM.

In general, the C implementation is always used so functools.reduce() doesn't accept keyword arguments. This change fix an inconsistency when the C extension (_functools) cannot be loaded.

ISTM that adding an argument restriction to an already published function is a breaking change.

I see how @Eclips4 would conclude that the likelihood of users running the Python version of functools.reduce is low. However, I have to agree with Nikita and Raymond here. It is unknowable whether users had or had not depended on keyword arguments here, especially after the function existing for so long. A lot of code isn't open.

In general, it might be tempting to treat a case like this one as a special case and relax our usual standards, but I believe it's not worth doing: we're not fixing a crippling problem, and we're not unblocking users to perform a legitimate operation. This change is a cleanup to improve consistency, it's not urgent.

Moreover, the function isn't actually documented as positional-only, so we cannot claim that the current behavior of the pure Python implementation was against stated intent.

Therefore, the responsible thing to do here would be to deprecate keyword argument use (through an awkward *args, **kwargs wrapper that would discover keyword usage and warn). Best case scenario, we might ask Thomas if he'd be okay with this new deprecation would be okay for 3.13 still, even though it's officially too late for that. But as Kirill says, the blast radius is limited as the function is shadowed by the C implementation for the vast majority of users.

Thanks Łukasz for sharing your thoughts!

Moreover, the function isn't actually documented as positional-only, so we cannot claim that the current behavior of the pure Python implementation was against stated intent.

Actually it's documented as positional-only, but only in 3.13+ branches:
https://docs.python.org/3.13/library/functools.html#functools.reduce

I absolutely agree with you that if we can deprecate it for 3.13, that would be fine. If not, that would also be fine (should we then revert the change to the docs mentioned above?).
cc @Yhg1s

No, I don't think this should be done in 3.13. The potential impact is low, but so is the benefit. It's fine to leave it documented as positional-only, but changes in behaviour (if warranted at all, and following the normal deprecation process) should go into 3.14 instead

PR was rewritten since I approved it

What's the status of this PR? I'm now confused :-(

I'm waiting for PR #125917 to be merged, so the initial argument would be pos-and-keyword. Therefore, I will change this PR to raise a DeprecationWarning if the Python version of functools.reduce is called with a function or sequence as a keyword argument. What do you think?

Now #125917 is merged. I think that @vstinner review should be dismissed as this pr reworked to allow initial kwarg.

Shouldn't we also add something in the "Deprecated" section of 3.14?

LGTM, except for missing comment on moved import.

I'm suggesting renaming _warn_kwargs to something more explicit for anyone stumbling upon the code. I'm also suggesting deleting the decorator from the module's global names.

I'm not sure if all my suggestions will pass the CI so just double-check before hitting the "commit suggestion" button if you want.

Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>

LGTM.

Thanks to all who participated! ❤️

…ntation of ``functools.reduce`` is called with `function` or `sequence` as a keyword args (python#121677) Python implementation of `functools` allows calling `reduce` with `function` or `sequence` as keyword args. This doesn't match behavior of our C accelerator and our documentation for `functools.reduce` states that `function`and `sequence` are positional-only arguments. Now calling a Python implementation of `functools.reduce` with `function` or `sequence` as keyword args would raise a `DeprecationWarning` and is planned to be prohibited in Python 3.16. Co-authored-by: Victor Stinner <vstinner@python.org> Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>

Fix signature of functools.reduce

73957ae

Eclips4 added needs backport to 3.12 only security fixes needs backport to 3.13 bugs and security fixes labels Jul 13, 2024

Eclips4 requested a review from vstinner July 13, 2024 10:31

Eclips4 requested a review from rhettinger as a code owner July 13, 2024 10:31

bedevere-app Bot mentioned this pull request Jul 13, 2024

Python implementation of functools.reduce accepts keyword arguments, while the C implementation does not #121676

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bedevere-app Bot added the awaiting review label Jul 13, 2024

sobolevn reviewed Jul 13, 2024

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Comment thread Lib/functools.py Outdated

vstinner previously approved these changes Jul 13, 2024

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bedevere-app Bot added awaiting merge and removed awaiting review labels Jul 13, 2024

Eclips4 removed the needs backport to 3.12 only security fixes label Jul 15, 2024

Eclips4 removed the needs backport to 3.13 bugs and security fixes label Jul 15, 2024

Raise a deprecation warning

dba4c4d

Eclips4 changed the title gh-121676: Fix the signature of the Python implementation of functools.reduce gh-121676: Raise a DeprecationWarning if the Python implementation of functools.reduce is called with a keyword args Jul 15, 2024

Eclips4 requested review from ambv, sobolevn and vstinner July 15, 2024 20:20

Eclips4 added 2 commits July 15, 2024 23:21

Fix versions

d81f9f1

Fix indent

6563f67

Eclips4 marked this pull request as draft July 15, 2024 20:58

bedevere-app Bot removed the awaiting merge label Jul 15, 2024

Move import right after the definition

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Eclips4 marked this pull request as ready for review July 15, 2024 21:26

bedevere-app Bot added the awaiting review label Jul 15, 2024

picnixz reviewed Oct 29, 2024

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Comment thread Misc/NEWS.d/next/Library/2024-07-13-13-25-31.gh-issue-121676.KDLS11.rst Outdated

skirpichev self-requested a review November 14, 2024 12:16

skirpichev added the DO-NOT-MERGE label Dec 6, 2024

Allow initial as keyword arg

991e330

Eclips4 removed the awaiting merge label Dec 28, 2024

Eclips4 requested review from picnixz and vstinner December 28, 2024 16:16

Fix typos

1cc1ce1

Eclips4 added awaiting review and removed DO-NOT-MERGE labels Dec 28, 2024

picnixz reviewed Dec 28, 2024

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Add an entry to the whatsnew

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Eclips4 requested a review from picnixz December 28, 2024 17:00

picnixz reviewed Dec 28, 2024

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Comment thread Doc/whatsnew/3.14.rst

skirpichev reviewed Dec 29, 2024

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Comment thread Lib/test/test_functools.py

Address review

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skirpichev approved these changes Dec 29, 2024

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Eclips4 requested a review from picnixz December 29, 2024 14:48

picnixz reviewed Dec 31, 2024

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Comment thread Lib/functools.py Outdated

Apply suggestions from code review

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Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>

picnixz self-requested a review January 1, 2025 10:54

picnixz approved these changes Jan 1, 2025

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Eclips4 merged commit d903b17 into python:main Jan 1, 2025

bedevere-app Bot removed the awaiting review label Jan 1, 2025

Eclips4 deleted the issue-121676 branch January 1, 2025 11:37

sobolevn mentioned this pull request May 8, 2026

Remove kw parameters from python version of functools.reduce #149537

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gh-121676: Raise a ``DeprecationWarning`` if the Python implementation of ``functools.reduce`` is called with a keyword args by Eclips4 · Pull Request #121677 · python/cpython · GitHub

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gh-121676: Raise a DeprecationWarning if the Python implementation of functools.reduce is called with a keyword args - #121677

gh-121676: Raise a DeprecationWarning if the Python implementation of functools.reduce is called with a keyword args#121677Eclips4 merged 14 commits intopython:mainpython/cpython:mainfrom Eclips4:issue-121676Eclips4/cpython:issue-121676Copy head branch name to clipboard

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gh-121676: Raise a DeprecationWarning if the Python implementation of functools.reduce is called with a keyword args#121677
Eclips4 merged 14 commits into
python:mainfrom
Eclips4:issue-121676

Eclips4 commented Jul 13, 2024 •
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Eclips4 commented Oct 29, 2024 •
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