Socket 是进程间通信的一种方式，它与其他进程间通信的一个主要不同是：它能实现不同主机间的进程间通信，我们网络上各种各样的服务大多都是基于 Socket 来完成通信的，例如我们每天浏览网页、QQ 聊天、收发 email 等等。要解决网络上两台主机之间的进程通信问题，首先要唯一标识该进程，在 TCP/IP 网络协议中，就是通过 (IP地址，协议，端口号) 三元组来标识进程的，解决了进程标识问题，就有了通信的基础了。
本文主要介绍使用Python 进行TCP Socket 网络编程，假设你已经具有初步的网络知识及Python 基本语法知识。
TCP 是一种面向连接的传输层协议，TCP Socket 是基于一种 Client-Server 的编程模型，服务端监听客户端的连接请求，一旦建立连接即可以进行传输数据。那么对 TCP Socket 编程的介绍也分为客户端和服务端：
一、客户端编程
创建socket
首先要创建 socket，用 Python 中 socket 模块的函数 socket 就可以完成：
#Socket client example in python import socket #for sockets #create an AF_INET, STREAM socket (TCP) s = socket.socket(socket.AF_INET, socket.SOCK_STREAM) print 'Socket Created'
函数socket.socket 创建一个 socket，返回该 socket 的描述符，将在后面相关函数中使用。该函数带有两个参数：
Address Family：可以选择 AF_INET（用于 Internet 进程间通信） 或者 AF_UNIX（用于同一台机器进程间通信）
Type：套接字类型，可以是 SOCKET_STREAM（流式套接字，主要用于 TCP 协议）或者SOCKET_DGRAM（数据报套接字，主要用于 UDP 协议）
注：由于本文主要概述一下 Python Socket 编程的过程，因此不会对相关函数参数、返回值进行详细介绍，需要了解的可以查看相关手册
错误处理
如果创建 socket 函数失败，会抛出一个 socket.error 的异常，需要捕获：
#handling errors in python socket programs import socket #for sockets import sys #for exit try: #create an AF_INET, STREAM socket (TCP) s = socket.socket(socket.AF_INET, socket.SOCK_STREAM) except socket.error, msg: print 'Failed to create socket. Error code: ' + str(msg[0]) + ' , Error message : ' + msg[1] sys.exit(); print 'Socket Created'
那么到目前为止已成功创建了 socket，接下来我们将用这个 socket 来连接某个服务器，就连 www.google.com 吧。
连接服务器
本文开始也提到了，socket 使用 (IP地址，协议，端口号) 来标识一个进程，那么我们要想和服务器进行通信，就需要知道它的 IP地址以及端口号。
获得远程主机的 IP 地址
Python 提供了一个简单的函数 socket.gethostbyname 来获得远程主机的 IP 地址：
host = 'www.google.com' port = 80 try: remote_ip = socket.gethostbyname( host ) except socket.gaierror: #could not resolve print 'Hostname could not be resolved. Exiting' sys.exit() print 'Ip address of ' + host + ' is ' + remote_ip
现在我们知道了服务器的 IP 地址，就可以使用连接函数 connect 连接到该 IP 的某个特定的端口上了，下面例子连接到 80 端口上（是 HTTP 服务的默认端口）：
#Connect to remote server s.connect((remote_ip , port)) print 'Socket Connected to ' + host + ' on ip ' + remote_ip
运行该程序：
$ python client.py Socket created Ip of remote host www.google.com is 173.194.38.145 Socket Connected to www.google.com on ip 173.194.38.145
发送数据
上面说明连接到 www.google.com 已经成功了，接下面我们可以向服务器发送一些数据，例如发送字符串GET / HTTP/1.1\r\n\r\n，这是一个 HTTP 请求网页内容的命令。
#Send some data to remote server message = "GET / HTTP/1.1\r\n\r\n" try : #Set the whole string s.sendall(message) except socket.error: #Send failed print 'Send failed' sys.exit() print 'Message send successfully'
发送完数据之后，客户端还需要接受服务器的响应。
接收数据
函数 recv 可以用来接收 socket 的数据：
#Now receive data reply = s.recv(4096) print reply
一起运行的结果如下：
Socket created Ip of remote host www.google.com is 173.194.38.145 Socket Connected to www.google.com on ip 173.194.38.145 Message send successfully HTTP/1.1 302 Found Cache-Control: private Content-Type: text/html; charset=UTF-8 Location: http://www.google.com.sg/?gfe_rd=cr&ei=PlqJVLCREovW8gfF0oG4CQ Content-Length: 262 Date: Thu, 11 Dec 2014 08:47:58 GMT Server: GFE/2.0 Alternate-Protocol: 80:quic,p=0.02 <HTML><HEAD><meta http-equiv="content-type" content="text/html;charset=utf-8"> <TITLE>302 Moved</TITLE></HEAD><BODY> <h1 id="Moved">302 Moved</h1> The document has moved <A HREF="http://www.google.com.sg/?gfe_rd=cr&ei=PlqJVLCREovW8gfF0oG4CQ">here</A>. </BODY></HTML>
关闭 socket
当我们不想再次请求服务器数据时，可以将该 socket 关闭，结束这次通信：
s.close()
小结
上面我们学到了如何：
- 创建 socket
- 连接到远程服务器
- 发送数据
- 接收数据
- 关闭 socket
当我们打开www.google.com 时，浏览器所做的就是这些，知道这些是非常有意义的。在 socket 中具有这种行为特征的被称为CLIENT，客户端主要是连接远程系统获取数据。
socket 中另一种行为称为SERVER，服务器使用 socket 来接收连接以及提供数据，和客户端正好相反。所以 www.google.com 是服务器，你的浏览器是客户端，或者更准确地说，www.google.com 是 HTTP 服务器，你的浏览器是 HTTP 客户端。
那么上面介绍了客户端的编程，现在轮到服务器端如果使用 socket 了。
二、服务器端编程
服务器端主要做以下工作：
- 打开 socket
- 绑定到特定的地址以及端口上
- 监听连接
- 建立连接
- 接收/发送数据
上面已经介绍了如何创建 socket 了，下面一步是绑定。
绑定socket
函数 bind 可以用来将 socket 绑定到特定的地址和端口上，它需要一个 sockaddr_in 结构作为参数：
import socket import sys HOST = '' # Symbolic name meaning all available interfaces PORT = 8888 # Arbitrary non-privileged port s = socket.socket(socket.AF_INET, socket.SOCK_STREAM) print 'Socket created' try: s.bind((HOST, PORT)) except socket.error , msg: print 'Bind failed. Error Code : ' + str(msg[0]) + ' Message ' + msg[1] sys.exit() print 'Socket bind complete'
绑定完成之后，接下来就是监听连接了。
监听连接
函数 listen 可以将 socket 置于监听模式：
s.listen(10) print 'Socket now listening'
该函数带有一个参数称为 backlog，用来控制连接的个数。如果设为 10，那么有 10 个连接正在等待处理，此时第 11 个请求过来时将会被拒绝。
接收连接
当有客户端向服务器发送连接请求时，服务器会接收连接：
#wait to accept a connection - blocking call conn, addr = s.accept() #display client information print 'Connected with ' + addr[0] + ':' + str(addr[1])
运行该程序的，输出结果如下：
$ python server.py Socket created Socket bind complete Socket now listening
此时，该程序在 8888 端口上等待请求的到来。不要关掉这个程序，让它一直运行，现在客户端可以通过该端口连接到 socket。我们用 telnet 客户端来测试，打开一个终端，输入 telnet localhost 8888：
$ telnet localhost 8888 Trying 127.0.0.1... Connected to localhost. Escape character is '^]'. Connection closed by foreign host.
这时服务端输出会显示：
$ python server.py Socket created Socket bind complete Socket now listening Connected with 127.0.0.1:59954
我们观察到客户端已经连接上服务器了。在建立连接之后，我们可以用来与客户端进行通信。下面例子演示的是，服务器建立连接之后，接收客户端发送来的数据，并立即将数据发送回去，下面是完整的服务端程序：
import socket import sys HOST = '' # Symbolic name meaning all available interfaces PORT = 8888 # Arbitrary non-privileged port s = socket.socket(socket.AF_INET, socket.SOCK_STREAM) print 'Socket created' try: s.bind((HOST, PORT)) except socket.error , msg: print 'Bind failed. Error Code : ' + str(msg[0]) + ' Message ' + msg[1] sys.exit() print 'Socket bind complete' s.listen(10) print 'Socket now listening' #wait to accept a connection - blocking call conn, addr = s.accept() print 'Connected with ' + addr[0] + ':' + str(addr[1]) #now keep talking with the client data = conn.recv(1024) conn.sendall(data) conn.close() s.close()
在一个终端中运行这个程序，打开另一个终端，使用 telnet 连接服务器，随便输入字符串，你会看到：
$ telnet localhost 8888 Trying 127.0.0.1... Connected to localhost. Escape character is '^]'. happy happy Connection closed by foreign host.
客户端（telnet）接收了服务器的响应。
我们在完成一次响应之后服务器立即断开了连接，而像www.google.com 这样的服务器总是一直等待接收连接的。我们需要将上面的服务器程序改造成一直运行，最简单的办法是将accept 放到一个循环中，那么就可以一直接收连接了。
保持服务
我们可以将代码改成这样让服务器一直工作：
import socket
import sys
HOST = '' # Symbolic name meaning all available interfaces
PORT = 5000 # Arbitrary non-privileged port
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
print 'Socket created'
try:
s.bind((HOST, PORT))
except socket.error , msg:
print 'Bind failed. Error Code : ' + str(msg[0]) + ' Message ' + msg[1]
sys.exit()
print 'Socket bind complete'
s.listen(10)
print 'Socket now listening'
#now keep talking with the client
while 1:
#wait to accept a connection - blocking call
conn, addr = s.accept()
print 'Connected with ' + addr[0] + ':' + str(addr[1])
data = conn.recv(1024)
reply = 'OK...' + data
if not data:
break
conn.sendall(reply)
conn.close()
s.close()
现在在一个终端下运行上面的服务器程序，再开启三个终端，分别用 telnet 去连接，如果一个终端连接之后不输入数据其他终端是没办法进行连接的，而且每个终端只能服务一次就断开连接。这从代码上也是可以看出来的。
这显然也不是我们想要的，我们希望多个客户端可以随时建立连接，而且每个客户端可以跟服务器进行多次通信，这该怎么修改呢？
处理连接
为了处理每个连接，我们需要将处理的程序与主程序的接收连接分开。一种方法可以使用线程来实现，主服务程序接收连接，创建一个线程来处理该连接的通信，然后服务器回到接收其他连接的逻辑上来。
import socket
import sys
from thread import *
HOST = '' # Symbolic name meaning all available interfaces
PORT = 8888 # Arbitrary non-privileged port
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
print 'Socket created'
#Bind socket to local host and port
try:
s.bind((HOST, PORT))
except socket.error , msg:
print 'Bind failed. Error Code : ' + str(msg[0]) + ' Message ' + msg[1]
sys.exit()
print 'Socket bind complete'
#Start listening on socket
s.listen(10)
print 'Socket now listening'
#Function for handling connections. This will be used to create threads
def clientthread(conn):
#Sending message to connected client
conn.send('Welcome to the server. Type something and hit enter\n') #send only takes string
#infinite loop so that function do not terminate and thread do not end.
while True:
#Receiving from client
data = conn.recv(1024)
reply = 'OK...' + data
if not data:
break
conn.sendall(reply)
#came out of loop
conn.close()
#now keep talking with the client
while 1:
#wait to accept a connection - blocking call
conn, addr = s.accept()
print 'Connected with ' + addr[0] + ':' + str(addr[1])
#start new thread takes 1st argument as a function name to be run, second is the tuple of arguments to the function.
start_new_thread(clientthread ,(conn,))
s.close()
再次运行上面的程序，打开三个终端来与主服务器建立 telnet 连接，这时候三个客户端可以随时接入，而且每个客户端可以与主服务器进行多次通信。
telnet 终端下可能输出如下：
$ telnet localhost 8888 Trying 127.0.0.1... Connected to localhost. Escape character is '^]'. Welcome to the server. Type something and hit enter hi OK...hi asd OK...asd cv OK...cv
要结束 telnet 的连接，按下 Ctrl-] 键，再输入 close 命令。
服务器终端的输出可能是这样的：
$ python server.py Socket created Socket bind complete Socket now listening Connected with 127.0.0.1:60730 Connected with 127.0.0.1:60731
到目前为止，我们学习了Python 下基本的socket 编程，之后还有相关文章向大家介绍，不要走开。
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