def METHOD_NAME(self):
    """Retrieve next line from helper."""
    while True:
        try:
            line = self.readline()
        except OSError as err:
            if err.errno == errno.EWOULDBLOCK or err.errno == errno.EAGAIN:
                return ""
            raise
        assert line or self.started, (
            "tcpdump did not start: %s" % self.last_line.strip()
        )
        if self.started:
            return line
        if re.search("listening on %s" % self.intf_name, line):
            self.started = True
            # When we see tcpdump start, then call provided functions.
            if self.funcs is not None:
                for func in self.funcs:
                    func()
        else:
            self.last_line = line