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