Proposal: add `@typing.format_specifiers`

This decorator factory would tell type checkers what format codes are supported by instances of that type. It would take variable positional arguments, each a string corresponding to a valid specification, and type checkers will know the class its returned decorator is applied to will only take those format specifications, so that it can flag mistyped format specifications and make subtle bugs clear. It, however, requires special-casing string formatting and the format builtin.

Currently it is possible to restrict the type of format_spec in __format__ to Literal[...], but that will cause Liskov substitution principle violations and type checkers may not consider it.

I am opening an issue here in hopes of getting more feedback because my DPO thread has gained little traction.

Beginne damit, den Vorschlag und den darin verlinkten DPO-Thread zu lesen, da keine Implementierungsdateien oder Tests identifiziert sind. Als Nächstes muss Einigkeit über das Verhalten des Decorators und die Handhabung durch den Type-Checker hergestellt werden; erledigt wäre dies mit einem akzeptierten Design, in dem die erforderlichen Sonderfälle definiert sind.