https://api.slack.com/methods/groups.setPurpose

def set_purpose(self, group_name, purpose):
        """ https://api.slack.com/methods/groups.setPurpose
        """
        group_id = self.get_group_id(group_name)
        self.params.update({
            'channel': group_id,
            'purpose': purpose,
            })
        return FromUrl('https://slack.com/api/groups.setPurpose', self._requests)(data=self.params).post()