Format specifiers are described in :func:`format_field` which is a
        static function.

def format_field(self, value, format_spec):
        """Format specifiers are described in :func:`format_field` which is a
        static function.
        """
        if format_spec:
            spec, arg = format_spec[0], format_spec[1:]
            arg = arg or None
        else:
            spec = arg = None
        return self._format_field(spec, arg, value, self.numeric_locale)