'Require at least one variable to allow ranking.'

def check_preconditions(self):

    self.Information.add_message('no_attributes', 'No    variables    to    rank.')
    self.Information.no_attributes.clear()
    data = self.master.discrete_data
    if ((not super().check_preconditions()) or (data is None)):
        return False
    if (not data.domain.attributes):
        self.Information.no_attributes()
        return False
    return True
