python`with .. as ..`聲明和多個返回值

Question

我試圖使用python with-statement（a.k.a.上下文管理器）來確保由server_socket.accept()創建的TCP連接套接字始終關閉。然而，明顯的形式不起作用，因爲accept()返回多個值。

有沒有辦法使用帶有多個返回值的函數with-statement？

下面是一個簡單的例子，我想使用類似註釋的代碼來替換try/finally塊。

#!/usr/bin/env python3 

import socket 
from socket import socket as Socket 

with Socket(socket.AF_INET, socket.SOCK_STREAM) as server_socket: 

     server_socket.bind(('', 8011)) 
     server_socket.listen(1) 
     print("server ready") 

     while True: 

      # with server_socket.accept() as (connection_socket, _): 
      #  request = connection_socket.recv(1024).decode('ascii') 
      #  reply = request.upper() 
      #  connection_socket.send(reply.encode('ascii')) 

      try: 
       connection_socket, _ = server_socket.accept() 
       request = connection_socket.recv(1024).decode('ascii') 
       reply = request.upper() 
       connection_socket.send(reply.encode('ascii')) 

      finally: 
       connection_socket.close() 

使用時的錯誤消息與評論語句是：

Traceback (most recent call last): 
    File "./test.py", line 26, in <module> 
    with server_socket.accept() as (connection_socket, _): 
AttributeError: __exit__ 

據推測，這是因爲一個元組不具有with所需的__exit__屬性。

Answer 1

socket.socket的返回值有一個內置的上下文管理器，它實現了__exit__和__enter__。通過accept返回的元組沒有，但有一個庫來添加：

import socket 
import contextlib 

@contextlib.contextmanager 
def accept(s): 
    c,a = s.accept() 
    print('client connected on',a) 
    yield c,a 
    print('client disconnected on',a) 
    c.close() 

with socket.socket() as s: 
    s.bind(('',8000)) 
    s.listen(1) 
    with accept(s) as (c,a): 
     while True: 
      data = c.recv(1024) 
      if not data: break 
      c.sendall(data) 

參見：

https://docs.python.org/3.3/library/contextlib.html

附：沒有contextlib：

import socket 

with socket.socket() as s: 
    s.bind(('',8000)) 
    s.listen(1) 
    with s.accept()[0] as c: 
     a = c.getpeername() 
     print('client connected on',a) 
     while True: 
      data = c.recv(1024) 
      if not data: break 
      c.sendall(data) 
     print('client disconnected on',a)