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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Undocumented handling of thousands separator for floats and differences wrt Decimal","articleBody":"# Bug report\n\n### Bug description:\n\nIn [docs](https://docs.python.org/3.14/library/string.html#format-specification-mini-language) we have: \n\u003e 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.\n\n\u003e 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.\n\nBut this doesn't specify how separators are inserted for customized alignment, consider:\n```pycon\n\u003e\u003e\u003e format(0.0, '\u003e020_f')\n'0000000000000.000000'\n\u003e\u003e\u003e format(0.0, '\u003c020_f')\n'0.000000000000000000'\n\u003e\u003e\u003e format(0.0, '=020_f')\n'0_000_000_000.000000'\n\u003e\u003e\u003e format(0.0, '^020_f')\n'0000000.000000000000'\n```\nProbably it's obvious, that separators might be inserted only in case of 0-padding and `=` alignment, though maybe this should be mentioned explicitly.\n\nNote also that neither from above supported by Decimal/Fraction, e.g.:\n```pycon\n\u003e\u003e\u003e format(Decimal(0.0), '=020_f')\nTraceback (most recent call last):\n  File \"\u003cpython-input-13\u003e\", line 1, in \u003cmodule\u003e\n    format(Decimal(0.0), '=020_f')\n    ~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^\nValueError: invalid format string\n\u003e\u003e\u003e format(Fraction(0.0), '=020_f')\n'            0.000000'\n\u003e\u003e\u003e format(float(Fraction(0.0)), '=020_f')\n'0_000_000_000.000000'\n```\nBTW, 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.\n\nEither this should be fixed (preferred) or documented.\n\n### CPython versions tested on:\n\nCPython main branch\n\n### Operating systems tested on:\n\n_No response_\n\n\u003c!-- gh-linked-prs --\u003e\n### Linked PRs\n* gh-131030\n* gh-131067\n* gh-132155\n* gh-136241\n* gh-136242\n* gh-136649\n\u003c!-- /gh-linked-prs --\u003e\n","author":{"url":"https://github.com/skirpichev","@type":"Person","name":"skirpichev"},"datePublished":"2025-02-28T07:23:23.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":2},"url":"https://github.com/130664/cpython/issues/130664"}