7-3 如何讓對象支持上下文管理協議，需要定義__enter__, __exit__方法 分別在with開始和結束時候被調用

7-3 如何讓對象支持上下文管理
解決方案:
實現上下文管理協議，需要定義enter, exit方法
他們分別在with開始和結束時候被調用

with TelnetClient2('127.0.0.1') as client:
    client.start()      

對象client是來自於__enter__函數的返回
最後退出的時候不管是否存在異常，都將會進入到__exit__處理

with open('test.txt','w') as fw:
    fw.write('hello,world')
    fw.writelines(['xyz\n',123\n'])
fw.close()

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_unt('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('> ')
        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 cleanup(self):
        self.tn.close()
        self.tn = None
        with open(self.addr + '_history.txt', 'w') as f:
            f.writeline(self.history)


class TelnetClient2(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('> ')
        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)
        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 TelnetClient2('127.0.0.1') as client:
    client.start()