gh-67790: Support basic formatting for Fraction by mdickinson · Pull Request #111320 · python/cpython · GitHub

PR #100161 added fancy float-style formatting for the Fraction type, but left us in a state where basic formatting for fractions (alignment, fill, thousands separators) still wasn't supported. For example, we can't currently specify a minimum width for a formatted fraction:

>>> format(Fraction(3, 2), '20')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.12/lib/python3.12/fractions.py", line 427, in __format__
    raise ValueError(
ValueError: Invalid format specifier '20' for object of type 'Fraction'

This PR adds that basic formatting support, aiming for compatibility with int formatting. The basic formatting is active either with presentation type d, or with no explicit presentation type. For example, on this branch:

>>> x = Fraction(103993, 33102)
>>> f"{x:.<20}"  # minimum width, fill and alignment
'103993/33102........'
>>> f"{x:_}"  # thousands separators
'103_993/33_102'
>>> f"{x:+}"  # sign specification
'+103993/33102'
>>> f"{x:+d}"  # explicit 'd' presentation type
'+103993/33102'
>>> y = Fraction(22)
>>> f"{y}"
'22'
>>> f"{y:#}"  # alternate flag '#' forces an explicit denominator
'22/1'

All of the above except f"{y}" currently give ValueError on main.

Some details:

Minimum width, fill character, alignment, the sign flag, and thousands separators are supported in the obvious way.
The zero-fill flag 0 is not supported (though it's still fine to use 0 as a fill character in the normal way): it's not 100% clear what the semantics should be (should both the numerator and denominator be padded? Just the numerator?) or what value it would provide, so until someone comes up with valid use-cases that can help inform the semantics, I'm leaving this unsupported.
The alternate flag # forces a slash and denominator in the formatted output, even when the output is integer-valued. By default, the slash and denominator are left off for integer-valued fractions.

Pinging @ericvsmith for awareness.

Issue: Implement __format__ for Fraction #67790

📚 Documentation preview 📚: https://cpython-previews--111320.org.readthedocs.build/

@serhiy-storchaka @ericvsmith Would either of you be interested in reviewing?

float.__format__ does not support d. On other hand, %d in printf-like formatting accept float, but truncate it. Would not it cause confusion? Users may (wrongly) suppose that the d format specifier always format a number as a decimal integer.

It is perhaps out of the scope of this PR or this issue, but it may be useful to provide options for:

output as improper fraction: 3/2 as 1 1/2.
always output a fraction slash: 3/1 as 3/1. (already possible with the # modifier)
specify different symbol for fraction slash, e.g. :, ÷ (U+00F7), ⁄ (U+2044), ∕ (U+2215), ∶ (U+2236).

@serhiy-storchaka Thank you for reviewing!

float.__format__ does not support d. On other hand, %d in printf-like formatting accept float, but truncate it. Would not it cause confusion?

Hmm, possibly. Now that I look at it, we're adding two ways to do exactly the same thing (that is, format(f, s) and format(f, s + 'd') do exactly the same thing where s is a format specifier without a presentation type). That seems unnecessary.

What do you think about simply dropping the d support for now? So for example format(f, '>.10') would still work but format(f, '>.10d') wouldn't. Then we can still add support for a d presentation type in the future if that looks useful.

it may be useful to provide options for [...]

Yes, I think I'd rather leave these to a separate PR.

What do you think about simply dropping the d support for now?

Looks fine to me.

LGTM

What do you think about simply dropping the d support for now? So for example format(f, '>.10') would still work but format(f, '>.10d') wouldn't. Then we can still add support for a d presentation type in the future if that looks useful.

Actually, I think I'd require the "d", just in case we want to do somethings else entirely without it. But I don't feel strongly about it.

OTOH, note that for ints, the 'd' has no effect.

@mdickinson It LGTM if drop 'd' and "integer-like".

Thanks @serhiy-storchaka and @ericvsmith for feedback.

@ericvsmith

Actually, I think I'd require the "d", just in case we want to do somethings else entirely without it.

Yes, I can see a case both ways, and part of me wants to require a presentation type for all non-trivial formatting. I think this is close enough to the obvious one way to do formatting for Fractions that I'm happy for it to be the "default" formatting, so not require a "d". I'll make that change.

LGTM. Thanks, Mark!

…hon 3.13. See python/cpython#111320

https://build.opensuse.org/request/show/1140281 by user dirkmueller + anag+factory - update to 1.16: * Formatting support was improved, following CPython 3.13a3 as of python/cpython#111320 * Add support for Python 3.13 by using Cython 3.0.8 and calling math.gcd().

PR python#100161 added fancy float-style formatting for the Fraction type, but left us in a state where basic formatting for fractions (alignment, fill, minimum width, thousands separators) still wasn't supported. This PR adds that support. --------- Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>

mdickinson added 3 commits October 25, 2023 11:48

pythongh-67790 : Add integer-style formatting for Fraction type

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Add docs

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Add news entry

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bedevere-app Bot mentioned this pull request Oct 25, 2023

Implement __format__ for Fraction #67790

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bedevere-app Bot added the awaiting core review label Oct 25, 2023

Add PR number

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mdickinson changed the title gh-67790: Support basic formatting in Fraction.__format__ gh-67790: Support basic formatting for Fraction Oct 25, 2023

Restore original line spacing in docs

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mdickinson mentioned this pull request Oct 25, 2023

gh-67790: Add integer-style formatting for fractions.Fraction mdickinson/cpython#2

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3 tasks

mdickinson added 2 commits October 25, 2023 15:16

Remove 'float-style', since it's now too specific

3e7b6b6

Tighten doc wording

acfa57c

serhiy-storchaka reviewed Oct 25, 2023

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serhiy-storchaka approved these changes Oct 25, 2023

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Comment thread Doc/whatsnew/3.13.rst Outdated

bedevere-app Bot added awaiting merge and removed awaiting core review labels Oct 25, 2023

ericvsmith approved these changes Oct 25, 2023

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Merge branch 'main' into fraction-format-d-presentation-type

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mdickinson added 3 commits December 3, 2023 09:55

Remove support for 'd' presentation type; don't refer to integer-style

8228182

Fix single backtick

2fc7d56

Fix grammar - 'or' is better than 'and' here

0df75d8

serhiy-storchaka approved these changes Dec 3, 2023

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Comment thread Doc/library/fractions.rst Outdated

mdickinson added 2 commits December 3, 2023 10:15

Tweak wording

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Update invalid format tests

30260f9

Fix a long line in the .rst source

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ericvsmith approved these changes Dec 6, 2023

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Merge branch 'main' into fraction-format-d-presentation-type

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mdickinson merged commit fe479fb into python:main Dec 16, 2023

bedevere-app Bot removed the awaiting merge label Dec 16, 2023

mdickinson deleted the fraction-format-d-presentation-type branch December 16, 2023 10:58

scoder added a commit to scoder/quicktions that referenced this pull request Jan 10, 2024

Implement more general string formatting for Fraction, following CPyt…

10c9793

…hon 3.13. See python/cpython#111320

gh-67790: Support basic formatting for Fraction by mdickinson · Pull Request #111320 · python/cpython · GitHub

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gh-67790: Support basic formatting for Fraction - #111320

gh-67790: Support basic formatting for Fraction#111320mdickinson merged 15 commits intopython:mainpython/cpython:mainfrom mdickinson:fraction-format-d-presentation-typemdickinson/cpython:fraction-format-d-presentation-typeCopy head branch name to clipboard

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gh-67790: Support basic formatting for Fraction#111320
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