:param fields:
            Set fields to substitute
        :returns:
            Substituted Template with given fields.
            If no fields were set up beforehand, :func:`raw` is used.

def sub(self):
        '''
        :param fields:
            Set fields to substitute
        :returns:
            Substituted Template with given fields.
            If no fields were set up beforehand, :func:`raw` is used.
        '''

        if self.__fields:
            return _Template(self.raw).substitute(self.__fields)
        return self.raw