Undocumented handling of thousands separator for floats and differences wrt Decimal

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 https://github.com/python/cpython/pull/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-131030
• gh-131067
• gh-132155
• gh-136241
• gh-136242
• gh-136649

Commencez par lire la documentation du format specification mini-language et reproduisez les exemples de float, Decimal et Fraction présents dans l’issue. Consultez les PRs liés pour comprendre le travail déjà en cours. Le travail est considéré comme terminé lorsque le comportement du séparateur et de l’alignement est résolu de manière cohérente ou que les règles observées sont documentées.