求帮忙解释下child.before,child.after

yongsheng_2012

def ssh_command (user, host, password, command):
    """
    This runs a command on the remote host. This could also be done with the
    pxssh class, but this demonstrates what that class does at a simpler level.
    This returns a pexpect.spawn object. This handles the case when you try to
    connect to a new host and ssh asks you if you want to accept the public key
    fingerprint and continue connecting.
    """
    ssh_newkey = 'Are you sure you want to continue connecting'
    # 为 ssh 命令生成一个 spawn 类的子程序对象.
    child = pexpect.spawn('ssh -l %s %s %s'%(user, host, command))
    i = child.expect([pexpect.TIMEOUT, ssh_newkey, 'password: '])
    # 如果登录超时，打印出错信息，并退出.
    if i == 0: # Timeout
        print 'ERROR!'
        print 'SSH could not login. Here is what SSH said:'
        print child.before, child.after
        return None
    # 如果 ssh 没有 public key，接受它.
    if i == 1: # SSH does not have the public key. Just accept it.
        child.sendline ('yes')
        child.expect ('password: ')
        i = child.expect([pexpect.TIMEOUT, 'password: '])
        if i == 0: # Timeout
        print 'ERROR!'
        print 'SSH could not login. Here is what SSH said:'
        print child.before, child.after
        return None
    # 输入密码.
    child.sendline(password)
    return child
麻烦大家帮忙解释下代码中的child.before，child.after，谢谢