小编mfu*_*man的帖子

I can use a custom class to extend Python's string formatting:

class CaseStr(str):
    def __format__(self, fmt):
        if fmt.endswith('u'):
            s = self.upper()
            fmt = fmt[:-1]
        elif fmt.endswith('l'):
            s = self.lower()
            fmt = fmt[:-1]
        else:
            s = str(self)
        return s.__format__(fmt)

I can then use this class to format arguments which are passed to a strings format method:

unformatted_string = 'uppercase: {s:u}, lowercase: {s:l}'
print unformatted_string.format(s=CaseStr('abc'))

While this works it seems awkward that the custom format specifier is in the base string, but …