CC@ericvsmith per experts index

Here is implementation pr, for context:#100161

The ValueError raised in case both alignment (and the fill character) and zero padding are specified on the ground "refuse the temptation to guess". In principle, it's not hard to reproduce float's behavior (zero padding is ignored if the fill character is specified). Though, maybe it's better to align this with the Fraction instead. Probably, it's a separate issue. Edit:#130716

Few another incompatibilities:#130664

As happens occasionally, the docs are out of sync with the implementation, and we should respond by updating the docs, not the code. IMO.

The problem is that the stdlib has several beasts (Decimal and Fraction), that have slightly different implementations of new-style formatting. IMO, documenting all these differences will make documentation much less readable.

Also, it's less obvious to which convention should follow external modules, like the mpmath or the gmpy2.

Okay, I will leave it to more active core devs to sort out.

Alternative pr:#130717