Subscribe to a channel

        :param channel_name: The name of the channel to subscribe to.
        :type channel_name: str

        :rtype : Channel

def subscribe(self, channel_name):
        """Subscribe to a channel

        :param channel_name: The name of the channel to subscribe to.
        :type channel_name: str

        :rtype : Channel
        """
        data = {'channel': channel_name}

        if channel_name.startswith('presence-'):
            data['auth'] = self._generate_presence_key(
                self.connection.socket_id,
                self.key,
                channel_name,
                self.secret,
                self.user_data
            )
            data['channel_data'] = json.dumps(self.user_data)
        elif channel_name.startswith('private-'):
            data['auth'] = self._generate_private_key(
                self.connection.socket_id,
                self.key,
                channel_name,
                self.secret
            )

        self.connection.send_event('pusher:subscribe', data)

        self.channels[channel_name] = Channel(channel_name, self.connection)

        return self.channels[channel_name]