def debug_msg_with_timestamp(strMessage=None):
    """
    return to caller a string message with timestamp for debug purpose

    @Args:
    * [strMessage] message to display, if none, the returning will just 
        only a timestamp of the format YYYY-mm-dd HH:MM:SS.ms

    @Returns:
    * [str] returning a string with timestamp
    """
    import datetime
    if strMessage is None:
        return datetime.datetime.now().strftime('%Y-%m-%d %H:%M:%S.%f')
    else:
        timestamp = datetime.datetime.now().strftime('%Y-%m-%d %H:%M:%S.%f')
        return '{0} {1}'.format(timestamp, strMessage)
