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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def channel(self, channel_id=None, synchronous=False):
 """ Fetch a Channel object identified by the numeric channel_id, or create that object if it doesn't already exist. If channel_id is not None but no channel exists for that id, will raise InvalidChannel. If there are already too many channels open, will raise TooManyChannels. If synchronous=True, then the channel will act synchronous in all cases where a protocol method supports `nowait=False`, or where there is an implied callback in the protocol. """

        if channel_id is None:
            # adjust for channel 0
            if len(self._channels) - 1 >= self._channel_max:
                raise Connection.TooManyChannels(
                    "%d channels already open, max %d",
                    len(self._channels) - 1,
                    self._channel_max)
            channel_id = self._next_channel_id()
            while channel_id in self._channels:
                channel_id = self._next_channel_id()
        elif channel_id in self._channels:
            return self._channels[channel_id]
        else:
            raise Connection.InvalidChannel(
                "%s is not a valid channel id", channel_id)

        # Call open() here so that ConnectionChannel doesn't have it called.
        # Could also solve this other ways, but it's a HACK regardless.
        rval = Channel(
            self, channel_id, self._class_map, synchronous=synchronous)
        self._channels[channel_id] = rval
        rval.add_close_listener(self._channel_closed)
        rval.open()
        return rval