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.

Empieza leyendo la propuesta y el hilo de DPO enlazado en ella, ya que no se han identificado archivos de implementación ni pruebas. El siguiente paso es llegar a un acuerdo sobre el comportamiento del decorador y el tratamiento por parte del comprobador de tipos; se consideraría terminado cuando exista un diseño aceptado con los casos especiales requeridos definidos.