convert a channel list to a string

        Args:
            channelList: channel list (i.e. [21, 22, 23])

        Returns:
            a comma separated channel string (i.e. '21, 22, 23')

def __ChannelMaskListToStr(self, channelList):
        """convert a channel list to a string

        Args:
            channelList: channel list (i.e. [21, 22, 23])

        Returns:
            a comma separated channel string (i.e. '21, 22, 23')
        """
        chan_mask_range = ''
        if isinstance(channelList, list):
            if len(channelList):
                chan_mask_range = ",".join(str(chan) for chan in channelList)
            else:
                print 'empty list'
        else:
            print 'not a valid channel list:', channelList

        return chan_mask_range