Description

skirpichev

opened on Feb 28, 2025

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Bug report

Bug description:

In docs we have:

The ',' option signals the use of a comma for a thousands separator for floating-point presentation types and for integer presentation type 'd'. For other presentation types, this option is an error. For a locale aware separator, use the 'n' integer presentation type instead.

The '_' option signals the use of an underscore for a thousands separator for floating-point presentation types and for integer presentation type 'd'. For integer presentation types 'b', 'o', 'x', and 'X', underscores will be inserted every 4 digits. For other presentation types, specifying this option is an error.

But this doesn't specify how separators are inserted for customized alignment, consider:

>>> format(0.0, '>020_f')
'0000000000000.000000'
>>> format(0.0, '<020_f')
'0.000000000000000000'
>>> format(0.0, '=020_f')
'0_000_000_000.000000'
>>> format(0.0, '^020_f')
'0000000.000000000000'

Probably it's obvious, that separators might be inserted only in case of 0-padding and = alignment, though maybe this should be mentioned explicitly.

Note also that neither from above supported by Decimal/Fraction, e.g.:

>>> format(Decimal(0.0), '=020_f')
Traceback (most recent call last):
  File "<python-input-13>", line 1, in <module>
    format(Decimal(0.0), '=020_f')
    ~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^
ValueError: invalid format string
>>> format(Fraction(0.0), '=020_f')
'            0.000000'
>>> format(float(Fraction(0.0)), '=020_f')
'0_000_000_000.000000'

BTW, the rationale for Fraction's behavior (see #100161) is "no special-casing of the particular '0=' fill-character/alignment combination", which is rather weak, IMO.

Either this should be fixed (preferred) or documented.

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

Linked PRs

• gh-130664: specify how thousands separators are used with 0-padding #131030
• gh-130664: treat '0' fill character with align '=' as zero-padding for Fraction's #131067
• gh-130664: support '_' (just as ',') in Decimal's formatting #132155
• [3.14] gh-130664: Treat '0' fill character with align '=' as zero-padding for Fraction's (GH-131067) #136241
• [3.13] gh-130664: Treat '0' fill character with align '=' as zero-padding for Fraction's (GH-131067) #136242
• [3.13] gh-130664: support '_' (just as ',') in Decimal's formatting (GH-132155) #136649

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1. skirpichev

   added

   docsDocumentation in the Doc dirDocumentation in the Doc dir

   extension-modulesC modules in the Modules dirC modules in the Modules dir

   interpreter-core(Objects, Python, Grammar, and Parser dirs)(Objects, Python, Grammar, and Parser dirs)

   stdlibStandard Library Python modules in the Lib/ directoryStandard Library Python modules in the Lib/ directory

   type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

   on Feb 28, 2025
2. skirpichev

   self-assigned this

   on Feb 28, 2025
3. github-project-automation

   added this to docs issueson Feb 28, 2025
4. github-project-automation

   moved this to Todo in docs issueson Feb 28, 2025
5. skirpichev

   added 2 commits that reference this issue on Mar 10, 2025

   pythongh-130664: specify how thousands separators are used with 0-pad…

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   pythongh-130664: treat '0' fill character with align '=' as zero-padd…

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6. skirpichev

   added a commit that references this issue on Apr 6, 2025

   pythongh-130664: support '_' (just as ',') in Decimal's formatting

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7. skirpichev

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8. skirpichev

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   interpreter-core(Objects, Python, Grammar, and Parser dirs)(Objects, Python, Grammar, and Parser dirs)

   on Apr 6, 2025
9. ambv

   added a commit that references this issue on Apr 15, 2025

   gh-130664: support '_' (just as ',') in Decimal's formatting (#132155)

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10. serhiy-storchaka commented on Jul 3, 2025

    

    serhiy-storchaka

    on Jul 3, 2025

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    There is also a wart: specifying the thousands separator with zero padding can increase the length of the resulting string over the specified width:

    >>> format(1, '04')
    '0001'
    >>> format(1, '04,')
    '0,001'
    >>> format(1, '4,')
    '   1'
11. serhiy-storchaka

    added a commit that references this issue on Jul 3, 2025

    gh-130664: Treat '0' fill character with align '=' as zero-padding fo…

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12. miss-islington

    added 2 commits that reference this issue on Jul 3, 2025

    pythongh-130664: Treat '0' fill character with align '=' as zero-padd…

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    pythongh-130664: Treat '0' fill character with align '=' as zero-padd…

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13. serhiy-storchaka

    added 2 commits that reference this issue on Jul 3, 2025

    [3.13] gh-130664: Treat '0' fill character with align '=' as zero-pad…

    c30178a

    [3.14] gh-130664: Treat '0' fill character with align '=' as zero-pad…

    4877f43
14. skirpichev commented on Jul 4, 2025

    

    skirpichev

    on Jul 4, 2025

    MemberAuthor

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    specifying the thousands separator with zero padding can increase the length of the resulting string over the specified width

    @serhiy-storchaka, I think we shouldn't add anything (after #131030). Docs says:

    The width is a decimal integer defining the minimum total field width, including any prefixes, separators, and other formatting characters.

    IMO, it's obvious, that as separators are placed between zeros (i.e. we can't start with '_' or ',') - the output string can be bigger from that.

    So, #131030 should finally fix issue.
15. AndPuQing

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    pythongh-130664: Treat '0' fill character with align '=' as zero-padd…

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16. Pranjal095

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    pythongh-130664: Treat '0' fill character with align '=' as zero-padd…

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17. picnixz

    added a commit that references this issue on Jul 13, 2025

    pythongh-130664: Treat '0' fill character with align '=' as zero-padd…

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18. miss-islington

    added a commit that references this issue on Jul 14, 2025

    pythongh-130664: support '_' (just as ',') in Decimal's formatting (p…

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19. picnixz

    added a commit that references this issue on Jul 14, 2025

    [3.13] gh-130664: support '_' (just as ',') in Decimal's formatting (G…

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20. picnixz

    marked , works with Decimal() against the doc of **Format Specification Mini-Language** #136636 as a duplicate of this issue on Jul 14, 2025
21. taegyunkim

    added a commit that references this issue on Aug 4, 2025

    pythongh-130664: Treat '0' fill character with align '=' as zero-padd…

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22. Agent-Hellboy

    added a commit that references this issue on Aug 19, 2025

    pythongh-130664: Treat '0' fill character with align '=' as zero-padd…

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