'Relay a message to the *other* party without needing to know
which party that is. This allows the calling command to not
have to worry about which party they are in the handler.
Args:
sender (object): One of A or B. The method will figure
out the *other* party to send to.
string (str): Text to send.'

def msg_other(self, sender, string):

    if (self.part_a == sender):
        self.part_b.msg(string)
    elif (self.part_b == sender):
        self.part_a.msg(string)
    else:
        (sender.msg(string) if sender else self.part_a.msg(string))
