让对象支持上下文管理

定义实例的__enter__，__exit__方法，它们分别在with开始和结束时被调用。

from telnetlib import Telnet
from sys import stdin, stdout
from collections import deque


class TelnetClient(object):
    def __init__(self, addr, port=23):
        self.addr = addr
        self.port = port
        self.tn = None
    def start(self):
        # user
        t = self.tn.read_until('login:')
        stdout.write(t)
        user = stdin.readline()
        self.tn.write(user)
        # password
        t = self.tn.read_until('Password:')
        if t.startswith(user[:-1]) : t = t[len(user)+1:]
        stdout.write(t)
        self.tn.write(stdin.readline())
        #
        t = self.tn.read_until('$ ')
        stdout.write(t)
        while True:
            uinput = stdin.readline()
            if not uinput:
                break
            self.history.append(uinput)
            self.tn.write(uinput)
            t = self.tn.read_until('$ ')
            stdout.write(t[len(uinput)+1:])
            
    def __enter__(self):
        self.tn = Telnet(self.addr, self.port)  # 连接对象
        self.history = deque()
        return self
    def __exit__(self, exc_type, exc_val, exc_tb):
        self.tn.close()
        self.tn = None
        with open(self.addr + '_history.txt', 'w')as f:
            f.writelines(self.history)
with TelnetClient('127.0.0.1')as client:
    client.start()
'''
client = TelnetClient('127.0.0.1')
print('\nstart...')
client.start()
print('\ncleanup')