def set_purpose(self, channel_name, purpose):
        """ https://api.slack.com/methods/channels.setPurpose
        """
        channel_id = self.get_channel_id(channel_name)
        self.params.update({
            'channel': channel_id,
            'purpose': purpose,
            })
        return FromUrl('https://slack.com/api/channels.setPurpose', self._requests)(data=self.params).post()