:param nick: nick or prefix

def get_user(self, nick: str) -> User:
        """
        :param nick: nick or prefix
        """
        hostmask = None
        if "!" in nick:
            nick, _, hostmask = nick.partition("!")
        user = self._users.get(nick)
        if not user:
            self._users[nick] = user = User(nick, self, hostmask=hostmask)
        elif not user.hostmask:
            user.hostmask = hostmask
        return user