gh-130662: accept leading zeros in precision/width for Fraction's formatting#130663

skirpichev

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

skirpichev

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 = 1/3
f"{x:_>010f}"  # '__3.140000', ignores the zero-pad
x = Fraction(1, 3)
Traceback (most recent call last):
  File "<python-input-4>", line 1, in <module>
    f"{x:_>010f}"
      ^^^^^^^^^^
  File "/Users/guido/cpython/Lib/fractions.py", line 600, in __format__
    raise ValueError(
    ...<2 lines>...
    )
ValueError: Invalid format specifier '_>010f' for object of 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

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

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

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

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

skirpichev

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

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

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

vstinner

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

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

vstinner

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

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

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

vstinner

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

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

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

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

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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skirpichev added the skip news label Feb 28, 2025

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

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serhiy-storchaka approved these changes Feb 28, 2025

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bedevere-app Bot added awaiting merge and removed awaiting review labels Feb 28, 2025

skirpichev commented Feb 28, 2025

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gvanrossum reviewed Feb 28, 2025

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Update Lib/test/test_fractions.py

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skirpichev mentioned this pull request Mar 1, 2025

gh-130662: refine width/precision specification in format-spec #130717

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vstinner approved these changes Apr 15, 2025

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skirpichev changed the title ~~gh-130662: make Fraction's formatting more compatible wrt float's~~ Apr 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

skirpichev requested a review from picnixz June 2, 2025 06:29

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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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skirpichev requested a review from picnixz June 2, 2025 11:39

picnixz approved these changes Jun 2, 2025

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

bedevere-app Bot removed the awaiting merge label Jun 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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Pranjal095 pushed a commit to Pranjal095/cpython that referenced this pull request Jul 12, 2025

pythongh-130662: Accept leading zeros in precision/width for Fraction… …

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

taegyunkim pushed a commit to taegyunkim/cpython that referenced this pull request Aug 4, 2025

pythongh-130662: Accept leading zeros in precision/width for Fraction… …

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

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