gh-130662: accept leading zeros in precision/width for Fraction's formatting by skirpichev · Pull Request #130663 · python/cpython

skirpichev · 2025-02-28T02:18:10Z

Issue: format(Fraction(1, 3), '.016f') raises ValueError #130662

skirpichev · 2025-02-28T03:59:42Z

CC @ericvsmith per experts index

serhiy-storchaka

LGTM.

I have some minor suggestions for tests.

gvanrossum

Note that there's another inconsistency between float and Fraction.

x=1/3f"{x:_>010f}"# '__3.140000', ignores the zero-padx=Fraction(1, 3)
Traceback (mostrecentcalllast):
File"<python-input-4>", line1, in<module>f"{x:_>010f}"^^^^^^^^^^File"/Users/guido/cpython/Lib/fractions.py", line600, in__format__raiseValueError(
...<2lines>...
)
ValueError: Invalidformatspecifier'_>010f'forobjectof type 'Fraction'

So we still fail the goal of Fraction supporting everything that float does. Since Fraction actually is more strict (which is the way of the future) aren't we fixing this the wrong way?

skirpichev · 2025-03-01T01:22:10Z

Here is implementation pr, for context: #100161

The ValueError raised in case both alignment (and the fill character) and zero padding are specified on the ground "refuse the temptation to guess". In principle, it's not hard to reproduce float's behavior (zero padding is ignored if the fill character is specified). Though, maybe it's better to align this with the Fraction instead. Probably, it's a separate issue. Edit: #130716

Few another incompatibilities: #130664

gvanrossum · 2025-03-01T03:20:27Z

As happens occasionally, the docs are out of sync with the implementation, and we should respond by updating the docs, not the code. IMO.

skirpichev · 2025-03-01T05:10:01Z

The problem is that the stdlib has several beasts (Decimal and Fraction), that have slightly different implementations of new-style formatting. IMO, documenting all these differences will make documentation much less readable.

Also, it's less obvious to which convention should follow external modules, like the mpmath or the gmpy2.

gvanrossum · 2025-03-01T20:03:51Z

Okay, I will leave it to more active core devs to sort out.

skirpichev · 2025-04-07T04:55:57Z

Alternative pr: #130717

vstinner

LGTM. I agree with @serhiy-storchaka: #130662 (comment)

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

miss-islington-app · 2025-06-02T13:28:24Z

Thanks @skirpichev for the PR, and @vstinner for merging it 🌮🎉.. I'm working now to backport this PR to: 3.14.
🐍🍒⛏🤖

…'s formatting (pythonGH-130663) (cherry picked from commit 5bc2d99) Co-authored-by: Sergey B Kirpichev <skirpichev@gmail.com> Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>

bedevere-app · 2025-06-02T13:28:34Z

GH-135026 is a backport of this pull request to the 3.14 branch.

vstinner · 2025-06-02T13:29:27Z

Merged, thank you.

GH-135026 is a backport of this pull request to the 3.14 branch.

I disagree: we should not backport this change to stable branches.

picnixz · 2025-06-02T13:30:23Z

Wasn't it a fix for something we introduced in 3.14? @skirpichev

vstinner · 2025-06-02T13:41:13Z

Wasn't it a fix for something we introduced in 3.14? @skirpichev

No, Python 3.13 has the same behavior for example. It was "always" like that.

@skirpichev proposed to have the behavior for all types: float, Fraction, Decimal. Before, only float accepted leading zeros.

skirpichev · 2025-06-02T13:56:20Z

I disagree: we should not backport this change to stable branches.

Why not? It's a bug, isn't?

vstinner · 2025-06-02T14:02:05Z

Why not? It's a bug, isn't?

From my point of view, it's a new feature and existing projects may rely on the current (Python 3.14) behavior.

skirpichev · 2025-06-02T14:23:26Z

From my point of view, it's a new feature and existing projects may rely on the current (Python 3.14) behavior.

How?!

The issue is about formatting stdlib types (Fractions and Decimals, in a separate pr). Should external projects use current formatting specification mini-language or use more strict rules? This was coming from mpmath/mpmath#915.

serhiy-storchaka · 2025-06-02T15:22:50Z

From my point of view, this is a borderline between a minor feature and an unsignificant bug fix which we may not want to backport. So maybe to 3.14 only? The solved problem is rather scholastic.

miss-islington-app · 2025-07-07T09:19:02Z

Thanks @skirpichev for the PR, and @vstinner for merging it 🌮🎉.. I'm working now to backport this PR to: 3.14.
🐍🍒⛏🤖

…'s formatting (pythonGH-130663) (cherry picked from commit 5bc2d99) Co-authored-by: Sergey B Kirpichev <skirpichev@gmail.com> Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>

bedevere-app · 2025-07-07T09:19:13Z

GH-136361 is a backport of this pull request to the 3.14 branch.

…n's formatting (GH-130663) (#136361) Co-authored-by: Sergey B Kirpichev <skirpichev@gmail.com> Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>

…'s formatting (python#130663) Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>

pythongh-130662: make Fraction's formatting more compatible wrt float's
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format(Fraction(1, 3), '.016f') raises ValueError #130662
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Update Lib/test/test_fractions.py
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gh-130662: refine width/precision specification in format-spec #130717
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skirpichev changed the title ~~gh-130662: make Fraction's formatting more compatible wrt float's~~gh-130662: accept leading zeros in precision/width for Fraction's formattingApr 15, 2025

skirpichev added the 3.14 bugs and security fixes label Apr 28, 2025

skirpichev added needs backport to 3.14 bugs and security fixes and removed 3.14 bugs and security fixes labels May 8, 2025

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Comment threadLib/test/test_fractions.py
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skirpichev added 2 commits June 2, 2025 14:25

Merge branch 'master' into fix-fmt-fractions/130662
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address review: add news
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vstinner merged commit 5bc2d99 into python:mainJun 2, 2025

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skirpichev deleted the fix-fmt-fractions/130662 branch June 2, 2025 13:56

skirpichev mentioned this pull request Jul 7, 2025
Add underscore as a decimal separator for string formatting #87790
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