def _to_string(self, fmt, locale=None):
        """
        Format the instance to a common string format.

        :param fmt: The name of the string format
        :type fmt: string

        :param locale: The locale to use
        :type locale: str or None

        :rtype: str
        """
        if fmt not in self._FORMATS:
            raise ValueError("Format [{}] is not supported".format(fmt))

        fmt = self._FORMATS[fmt]
        if callable(fmt):
            return fmt(self)

        return self.format(fmt, locale=locale)