Pull apart the format: [[fill]align][0][width][.precision][type]

def extract_format_spec(cls, format):
        """Pull apart the format: [[fill]align][0][width][.precision][type]"""
        # -- BASED-ON: parse.extract_format()
        # pylint: disable=redefined-builtin, unsubscriptable-object
        if not format:
            raise ValueError("INVALID-FORMAT: %s (empty-string)" % format)

        orig_format = format
        fill = align = None
        if format[0] in cls.ALIGN_CHARS:
            align = format[0]
            format = format[1:]
        elif len(format) > 1 and format[1] in cls.ALIGN_CHARS:
            fill = format[0]
            align = format[1]
            format = format[2:]

        zero = False
        if format and format[0] == '0':
            zero = True
            format = format[1:]

        width = ''
        while format:
            if not format[0].isdigit():
                break
            width += format[0]
            format = format[1:]

        precision = None
        if format.startswith('.'):
            # Precision isn't needed but we need to capture it so that
            # the ValueError isn't raised.
            format = format[1:]  # drop the '.'
            precision = ''
            while format:
                if not format[0].isdigit():
                    break
                precision += format[0]
                format = format[1:]

        # the rest is the type, if present
        type = format
        if not type:
            raise ValueError("INVALID-FORMAT: %s (without type)" % orig_format)
        return FormatSpec(type, width, zero, align, fill, precision)