Aye, definitely worth a thread on python-ideas. My rationale for suggesting something based on the built-in numeric codes is that it makes it straightforward for *users* to transfer knowledge from that mini-language.

As far as parsing goes, I was thinking of something along the lines of the following naive approach:

    typechar = fmt[-1:]
    if not typechar or typechar not in ("b", "n", "x"):
        return super().__format__(fmt)
    prefix, group_sep, suffix = fmt[:-1].rpartition("_")
    if prefix and prefix != "#" or suffix:
        return super().__format__(fmt)
    field_width = self._calculate_field_width(typechar)
    return format(int(self), f"{prefix}0{field_width}{group_sep}{type_char}")