Fetch a Channel object identified by the numeric channel_id, or
        create that object if it doesn't already exist.

def channel(self, channel_id=None):
        """
        Fetch a Channel object identified by the numeric channel_id, or
        create that object if it doesn't already exist.

        """
        if channel_id in self.channels:
            return self.channels[channel_id]

        return Channel(self, channel_id)