Python之freshman08 Socket

1. Socket介绍

概念

A network socket is an endpoint of a connection across a computer network. Today, most communication between computers is based on the Internet Protocol; therefore most network sockets are Internet sockets. More precisely, a socket is a handle (abstract reference) that a local program can pass to the networking application programming interface (API) to use the connection, for example "send this data on this socket".

实现一个socket至少要分以下几步，(伪代码)

 Socket socket = getSocket(type = "TCP")  #设定好协议类型
 connect(socket, address = "1.2.3.4", port = "") #连接远程机器
 send(socket, "Hello, world!") #发送消息
 close(socket) #关闭连接

A socket API is an application programming interface (API), usually provided by the operating system, that allows application programs to control and use network sockets. Internet socket APIs are usually based on the Berkeley sockets standard. In the Berkeley sockets standard, sockets are a form of file descriptor (a file handle), due to the Unix philosophy that "everything is a file", and the analogies between sockets and files: you can read, write, open, and close both. 　　

A socket address is the combination of an IP address and a port number, much like one end of a telephone connection is the combination of a phone number and a particular extension. Sockets need not have an address (for example for only sending data), but if a program binds a socket to an address, the socket can be used to receive data sent to that address. Based on this address, internet sockets deliver incoming data packets to the appropriate application process or thread.

Socket Families(地址簇)

socket.AF_UNIX unix本机进程间通信 

socket.AF_INET　IPV4　

socket.AF_INET6  IPV6

These constants represent the address (and protocol) families, used for the first argument to socket(). If the AF_UNIX constant is not defined then this protocol is unsupported. More constants may be available depending on the system.

Socket Types

socket.SOCK_STREAM  #for tcp

socket.SOCK_DGRAM   #for udp 

socket.SOCK_RAW     #原始套接字，普通的套接字无法处理ICMP、IGMP等网络报文，而SOCK_RAW可以；其次，SOCK_RAW也可以处理特殊的IPv4报文；此外，利用原始套接字，可以通过IP_HDRINCL套接字选项由用户构造IP头。

socket.SOCK_RDM  #是一种可靠的UDP形式，即保证交付数据报但不保证顺序。SOCK_RAM用来提供对原始协议的低级访问，在需要执行某些特殊操作时使用，如发送ICMP报文。SOCK_RAM通常仅限于高级用户或管理员运行的程序使用。

socket.SOCK_SEQPACKET #废弃了

These constants represent the socket types, used for the second argument to socket(). More constants may be available depending on the system. (Only SOCK_STREAM and SOCK_DGRAM appear to be generally useful.)

2. Socket 参数介绍

socket.socket(family=AF_INET, type=SOCK_STREAM, proto=0, fileno=None)  必会

Create a new socket using the given address family, socket type and protocol number. The address family should be AF_INET (the default), AF_INET6, AF_UNIX, AF_CAN or AF_RDS. The socket type should beSOCK_STREAM (the default), SOCK_DGRAM, SOCK_RAW or perhaps one of the other SOCK_ constants. The protocol number is usually zero and may be omitted or in the case where the address family is AF_CAN the protocol should be one of CAN_RAW or CAN_BCM. If fileno is specified, the other arguments are ignored, causing the socket with the specified file descriptor to return. Unlike socket.fromfd(), fileno will return the same socket and not a duplicate. This may help close a detached socket using socket.close().

socket.socketpair([family[, type[, proto]]])

Build a pair of connected socket objects using the given address family, socket type, and protocol number. Address family, socket type, and protocol number are as for the socket() function above. The default family is AF_UNIX if defined on the platform; otherwise, the default is AF_INET.

socket.create_connection(address[, timeout[, source_address]])

Connect to a TCP service listening on the Internet address (a 2-tuple (host, port)), and return the socket object. This is a higher-level function than socket.connect(): if host is a non-numeric hostname, it will try to resolve it for both AF_INET and AF_INET6, and then try to connect to all possible addresses in turn until a connection succeeds. This makes it easy to write clients that are compatible to both IPv4 and IPv6.

Passing the optional timeout parameter will set the timeout on the socket instance before attempting to connect. If no timeout is supplied, the global default timeout setting returned by getdefaulttimeout() is used.

If supplied, source_address must be a 2-tuple (host, port) for the socket to bind to as its source address before connecting. If host or port are ‘’ or 0 respectively the OS default behavior will be used.

socket.getaddrinfo(host, port, family=0, type=0, proto=0, flags=0) #获取要连接的对端主机地址 必会

sk.bind(address) 必会

　　s.bind(address) 将套接字绑定到地址。address地址的格式取决于地址族。在AF_INET下，以元组（host,port）的形式表示地址。

sk.listen(backlog) 必会

　　开始监听传入连接。backlog指定在拒绝连接之前，可以挂起的最大连接数量。

backlog等于5，表示内核已经接到了连接请求，但服务器还没有调用accept进行处理的连接个数最大为5
      这个值不能无限大，因为要在内核中维护连接队列

sk.setblocking(bool) 必会

　　是否阻塞（默认True），如果设置False，那么accept和recv时一旦无数据，则报错。

sk.accept() 必会

　　接受连接并返回（conn,address）,其中conn是新的套接字对象，可以用来接收和发送数据。address是连接客户端的地址。

　　接收TCP 客户的连接（阻塞式）等待连接的到来

sk.connect(address) 必会

　　连接到address处的套接字。一般，address的格式为元组（hostname,port）,如果连接出错，返回socket.error错误。

sk.connect_ex(address)

　　同上，只不过会有返回值，连接成功时返回 0 ，连接失败时候返回编码，例如：10061

sk.close() 必会

　　关闭套接字

sk.recv(bufsize[,flag]) 必会

　　接受套接字的数据。数据以字符串形式返回，bufsize指定最多可以接收的数量。flag提供有关消息的其他信息，通常可以忽略。

sk.recvfrom(bufsize[.flag])

　　与recv()类似，但返回值是（data,address）。其中data是包含接收数据的字符串，address是发送数据的套接字地址。

sk.send(string[,flag]) 必会

　　将string中的数据发送到连接的套接字。返回值是要发送的字节数量，该数量可能小于string的字节大小。即：可能未将指定内容全部发送。

sk.sendall(string[,flag]) 必会

　　将string中的数据发送到连接的套接字，但在返回之前会尝试发送所有数据。成功返回None，失败则抛出异常。

内部通过递归调用send，将所有内容发送出去。

sk.sendto(string[,flag],address)

　　将数据发送到套接字，address是形式为（ipaddr，port）的元组，指定远程地址。返回值是发送的字节数。该函数主要用于UDP协议。

sk.settimeout(timeout) 必会

　　设置套接字操作的超时期，timeout是一个浮点数，单位是秒。值为None表示没有超时期。一般，超时期应该在刚创建套接字时设置，因为它们可能用于连接的操作（如 client 连接最多等待5s ）

sk.getpeername()  必会

　　返回连接套接字的远程地址。返回值通常是元组（ipaddr,port）。

sk.getsockname() 

　　返回套接字自己的地址。通常是一个元组(ipaddr,port)

sk.fileno() 

　　套接字的文件描述符

socket.sendfile(file, offset=0, count=None)

     发送文件 ，但目前多数情况下并无什么卵用。

3.基本socket实例

 import socket

 server = socket.socket() #获得socket实例

 server.bind(("localhost",9998)) #绑定ip port
 server.listen()  #开始监听
 print("等待客户端的连接...")
 conn,addr = server.accept() #接受并建立与客户端的连接,程序在此处开始阻塞,只到有客户端连接进来...
 print("新连接:",addr )

 data = conn.recv(1024)
 print("收到消息:",data)

 server.close()

 SocketServer.py

 import socket

 client = socket.socket()

 client.connect(("localhost",9998))

 client.send(b"hey")

 client.close()

 SocketClient.py

多次的数据交互

 import socket

 server = socket.socket() #获得socket实例

 server.bind(("localhost",9998)) #绑定ip port
 server.listen()  #开始监听
 print("等待客户端的连接...")
 conn,addr = server.accept() #接受并建立与客户端的连接,程序在此处开始阻塞,只到有客户端连接进来...
 print("新连接:",addr )
 while True:

     data = conn.recv(1024)

     print("收到消息:",data)
     conn.send(data.upper())

 server.close()

 socketserver 支持多次交互

 import socket

 client = socket.socket()

 client.connect(("localhost",9998))

 while True:
     msg = input(">>:").strip()
     if len(msg) == 0:continue
     client.send( msg.encode("utf-8") )

     data = client.recv(1024)
     print("来自服务器:",data)

 client.close()

 socket客户端支持多交互

判断客户端是否断开

 import socket

 server = socket.socket() #获得socket实例

 server.bind(("localhost",9998)) #绑定ip port
 server.listen()  #开始监听
 print("等待客户端的连接...")
 conn,addr = server.accept() #接受并建立与客户端的连接,程序在此处开始阻塞,只到有客户端连接进来...
 print("新连接:",addr )
 while True:

     data = conn.recv(1024)
     if not data:
         print("客户端断开了...")
         break
     print("收到消息:",data)
     conn.send(data.upper())

 server.close()

 加了判断客户端是否断开的代码

4.Socket实现多连接处理

上面的代码虽然实现了服务端与客户端的多次交互，但是你会发现，如果客户端断开了， 服务器端也会跟着立刻断开，因为服务器只有一个while 循环，客户端一断开，服务端收不到数据 ，就会直接break跳出循环，然后程序就退出了，这显然不是我们想要的结果 ，我们想要的是，客户端如果断开了，我们这个服务端还可以为下一个客户端服务，它不能断，她接完一个客，擦完嘴角的遗留物，就要接下来勇敢的去接待下一个客人。 在这里如何实现呢？

1
conn,addr = server.accept() #接受并建立与客户端的连接,程序在此处开始阻塞,只到有客户端连接进来...

我们知道上面这句话负责等待并接收新连接，对于上面那个程序，其实在while break之后，只要让程序再次回到上面这句代码这，就可以让服务端继续接下一个客户啦。

 import socket

 server = socket.socket() #获得socket实例

 server.bind(("localhost",9998)) #绑定ip port
 server.listen()  #开始监听

 while True: #第一层loop
     print("等待客户端的连接...")
     conn,addr = server.accept() #接受并建立与客户端的连接,程序在此处开始阻塞,只到有客户端连接进来...
     print("新连接:",addr )
     while True:

         data = conn.recv(1024)
         if not data:
             print("客户端断开了...")
             break #这里断开就会再次回到第一次外层的loop
         print("收到消息:",data)
         conn.send(data.upper())

 server.close()

5.通过socket实现简单的ssh

光只是简单的发消息、收消息没意思，干点正事，可以做一个极简版的ssh，就是客户端连接上服务器后，让服务器执行命令，并返回结果给客户端。

 import socket
 import os

 server = socket.socket() #获得socket实例
 #server.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)

 server.bind(("localhost",9998)) #绑定ip port
 server.listen()  #开始监听

 while True: #第一层loop
     print("等待客户端的连接...")
     conn,addr = server.accept() #接受并建立与客户端的连接,程序在此处开始阻塞,只到有客户端连接进来...
     print("新连接:",addr )
     while True:

         data = conn.recv(1024)
         if not data:
             print("客户端断开了...")
             break #这里断开就会再次回到第一次外层的loop
         print("收到命令:",data)
         res = os.popen(data.decode()).read() #py3 里socket发送的只有bytes,os.popen又只能接受str,所以要decode一下
         print(len(res))
         conn.send(res.encode("utf-8"))

 server.close()

 socket ssh server

 import socket

 client = socket.socket()

 client.connect(("localhost",9998))

 while True:
     msg = input(">>:").strip()
     if len(msg) == 0:continue
     client.send( msg.encode("utf-8") )

     data = client.recv(1024)
     print(data.decode()) #命令执行结果

 client.close()

 socket ssh client

very cool , 这样我们就做了一个简单的ssh , 但多试几条命令你就会发现，上面的程序有以下2个问题。

1. 不能执行top等类似的 会持续输出的命令,这是因为，服务器端在收到客户端指令后，会一次性通过os.popen执行，并得到结果后返回给客户，但top这样的命令用os.popen执行你会发现永远都不会结束，所以客户端也永远拿不到返回。(真正的ssh是通过select 异步等模块实现的，我们以后会涉及)
2. 不能执行像cd这种没有返回的指令， 因为客户端每发送一条指令，就会通过client.recv(1024)等待接收服务器端的返回结果，但是cd命令没有结果 ，服务器端调用conn.send(data)时是不会发送数据给客户端的。 所以客户端就会一直等着，等到天荒地老，结果就卡死了。解决的办法是，在服务器端判断命令的执行返回结果的长度，如果结果为空，就自己加个结果返回给客户端，如写上"cmd exec success, has no output."
3. 如果执行的命令返回结果的数据量比较大，会发现，结果返回不全，在客户端上再执行一条命令，结果返回的还是上一条命令的后半段的执行结果，这是为什么呢？这是因为，我们的客户写client.recv(1024)， 即客户端一次最多只接收1024个字节，如果服务器端返回的数据是2000字节，那有至少9百多字节是客户端第一次接收不了的，那怎么办呢，服务器端此时不能把数据直接扔了呀，so它会暂时存在服务器的io发送缓冲区里，等客户端下次再接收数据的时候再发送给客户端。 这就是为什么客户端执行第2条命令时，却接收到了第一条命令的结果的原因。 这时有同学说了， 那我直接在客户端把client.recv(1024)改大一点不就好了么， 改成一次接收个100mb,哈哈，这是不行的，因为socket每次接收和发送都有最大数据量限制的，毕竟网络带宽也是有限的呀，不能一次发太多，发送的数据最大量的限制 就是缓冲区能缓存的数据的最大量，这个缓冲区的最大值在不同的系统上是不一样的， 我实在查不到一个具体的数字，但测试的结果是，在linux上最大一次可接收10mb左右的数据，不过官方的建议是不超过8k,也就是8192，并且数据要可以被2整除，不要问为什么 。anyway , 如果一次只能接收最多不超过8192的数据 ，那服务端返回的数据超过了这个数字怎么办呢？比如让服务器端打开一个5mb的文件并返回，客户端怎么才能完整的接受到呢？那就只能循环收取啦。

在开始解决上面问题3之前，我们要考虑，客户端要循环接收服务器端的大量数据返回直到一条命令的结果全部返回为止， 但问题是客户端知道服务器端返回的数据有多大么？答案是不知道，那既然不知道服务器的要返回多大的数据，那客户端怎么知道要循环接收多少次呢？答案是不知道，擦，那咋办？ 总不能靠猜吧？呵呵。。。 当然不能，那只能让服务器在发送数据之前主动告诉客户端，要发送多少数据给客户端，然后再开始发送数据，yes, 机智如我，搞起。

先简单测试接收数据量大小

 import socket
 import os,subprocess

 server = socket.socket() #获得socket实例
 server.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)

 server.bind(("localhost",9998)) #绑定ip port
 server.listen()  #开始监听

 while True: #第一层loop
     print("等待客户端的连接...")
     conn,addr = server.accept() #接受并建立与客户端的连接,程序在此处开始阻塞,只到有客户端连接进来...
     print("新连接:",addr )
     while True:

         data = conn.recv(1024)
         if not data:
             print("客户端断开了...")
             break #这里断开就会再次回到第一次外层的loop
         print("收到命令:",data)
         #res = os.popen(data.decode()).read() #py3 里socket发送的只有bytes,os.popen又只能接受str,所以要decode一下
         res = subprocess.Popen(data,shell=True,stdout=subprocess.PIPE).stdout.read() #跟上面那条命令的效果是一样的
         if len(res) == 0:
             res = "cmd exec success,has not output!"
         conn.send(str(len(res)).endcode("utf-8")) #发送数据之前,先告诉客户端要发多少数据给它
         conn.sendall(res.encode("utf-8")) #发送端也有最大数据量限制,所以这里用sendall,相当于重复循环调用conn.send,直至数据发送完毕

 server.close()

 ssh server 返回执行结果大小

 import socket

 client = socket.socket()

 client.connect(("localhost",9998))

 while True:
     msg = input(">>:").strip()
     if len(msg) == 0:continue
     client.send( msg.encode("utf-8") )

     res_return_size  = client.recv(1024) #接收这条命令执行结果的大小
     print("getting cmd result , ", res_return_size)
     total_rece_size = int(res_return_size)
     print(total_rece_size)

     #print(data.decode()) #命令执行结果

 client.close()

 ssh client 接收执行结果的大小

“粘包”， 即服务器端你调用时send 2次，但你send调用时，数据其实并没有立刻被发送给客户端，而是放到了系统的socket发送缓冲区里，等缓冲区满了、或者数据等待超时了，数据才会被send到客户端，这样就把好几次的小数据拼成一个大数据，统一发送到客户端了，这么做的目地是为了提高io利用效率，一次性发送总比连发好几次效率高嘛。 但也带来一个问题，就是“粘包”，即2次或多次的数据粘在了一起统一发送了。就是我们上面看到的情况 。

我们在这里必须要想办法把粘包分开， 因为不分开，你就没办法取出来服务器端返回的命令执行结果的大小呀。so ,那怎么分开呢？首先你是没办法让缓冲区强制刷新把数据发给客户端的。 你能做的，只有一个。就是，让缓冲区超时，超时了，系统就不会等缓冲区满了，会直接把数据发走，因为不能一个劲的等后面的数据呀，等太久，会造成数据延迟了，那可是极不好的。so如果让缓冲区超时呢？

答案就是：

1. time.sleep(0.5),经多次测试，让服务器程序sleep 至少0.5就会造成缓冲区超时。哈哈哈， 你会说，擦，这么玩不会被老板开除么，虽然我们觉得0.5s不多，但是对数据实时要求高的业务场景，比如股票交易，过了0.5s 股票价格可以就涨跌很多，搞毛线呀。但没办法，我刚学socket的时候 找不到更好的办法，就是这么玩的，现在想想也真是low呀
2. 但现在我是有Tesla的男人了，不能再这么low了， 所以推出nb新姿势就是， 不用sleep,服务器端每发送一个数据给客户端，就立刻等待客户端进行回应，即调用 conn.recv(1024), 由于recv在接收不到数据时是阻塞的，这样就会造成，服务器端接收不到客户端的响应，就不会执行后面的conn.sendall(命令结果)的指令，收到客户端响应后，再发送命令结果时，缓冲区就已经被清空了，因为上一次的数据已经被强制发到客户端了。 好机智 ， 看下面代码实现。

 #_*_coding:utf-8_*_
 __author__ = 'Alex Li'

 import socket
 import os,subprocess

 server = socket.socket() #获得socket实例
 server.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)

 server.bind(("localhost",9999)) #绑定ip port
 server.listen()  #开始监听

 while True: #第一层loop
     print("等待客户端的连接...")
     conn,addr = server.accept() #接受并建立与客户端的连接,程序在此处开始阻塞,只到有客户端连接进来...
     print("新连接:",addr )
     while True:

         data = conn.recv(1024)
         if not data:
             print("客户端断开了...")
             break #这里断开就会再次回到第一次外层的loop
         print("收到命令:",data)
         #res = os.popen(data.decode()).read() #py3 里socket发送的只有bytes,os.popen又只能接受str,所以要decode一下
         res = subprocess.Popen(data,shell=True,stdout=subprocess.PIPE).stdout.read() #跟上面那条命令的效果是一样的
         if len(res) == 0:
             res = "cmd exec success,has not output!".encode("utf-8")
         conn.send(str(len(res)).encode("utf-8")) #发送数据之前,先告诉客户端要发多少数据给它
         print("等待客户ack应答...")
         client_final_ack = conn.recv(1024) #等待客户端响应
         print("客户应答:",client_final_ack.decode())
         print(type(res))
         conn.sendall(res) #发送端也有最大数据量限制,所以这里用sendall,相当于重复循环调用conn.send,直至数据发送完毕

 server.close()

 接收大数据 server端

 #_*_coding:utf-8_*_
 __author__ = 'Alex Li'

 import socket
 import sys

 client = socket.socket()

 client.connect(("localhost",9999))

 while True:
     msg = input(">>:").strip()
     if len(msg) == 0:continue
     client.send( msg.encode("utf-8") )

     res_return_size  = client.recv(1024) #接收这条命令执行结果的大小
     print("getting cmd result , ", res_return_size)
     total_rece_size = int(res_return_size)
     print("total size:",res_return_size)
     client.send("准备好接收了,发吧loser".encode("utf-8"))
     received_size = 0 #已接收到的数据
     cmd_res = b''
     f = open("test_copy.html","wb")#把接收到的结果存下来,一会看看收到的数据 对不对
     while received_size != total_rece_size: #代表还没收完
         data = client.recv(1024)
         received_size += len(data) #为什么不是直接1024,还判断len干嘛,注意,实际收到的data有可能比1024少
         cmd_res += data
     else:
         print("数据收完了",received_size)
         #print(cmd_res.decode())
         f.write(cmd_res) #把接收到的结果存下来,一会看看收到的数据 对不对
     #print(data.decode()) #命令执行结果

 client.close()

 接收大数据客户端

6. SocketServer

The socketserver module simplifies the task of writing network servers.

socketserver一共有这么几种类型

1	class socketserver.TCPServer(server_address, RequestHandlerClass, bind_and_activate=True)

This uses the Internet TCP protocol, which provides for continuous streams of data between the client and server.

1	class socketserver.UDPServer(server_address, RequestHandlerClass, bind_and_activate=True)

This uses datagrams, which are discrete packets of information that may arrive out of order or be lost while in transit. The parameters are the same as for TCPServer.

1 2	class socketserver.UnixStreamServer(server_address, RequestHandlerClass, bind_and_activate=True) class socketserver.UnixDatagramServer(server_address, RequestHandlerClass,bind_and_activate=True)

These more infrequently used classes are similar to the TCP and UDP classes, but use Unix domain sockets; they’re not available on non-Unix platforms. The parameters are the same as for TCPServer.

There are five classes in an inheritance diagram, four of which represent synchronous servers of four types:

+------------+
| BaseServer |
+------------+
      |
      v
+-----------+        +------------------+
| TCPServer |------->| UnixStreamServer |
+-----------+        +------------------+
      |
      v
+-----------+        +--------------------+
| UDPServer |------->| UnixDatagramServer |
+-----------+        +--------------------+

创建一个socketserver 至少分以下几步:

1. First, you must create a request handler class by subclassing the BaseRequestHandlerclass and overriding its handle() method; this method will process incoming requests. 　　
2. Second, you must instantiate one of the server classes, passing it the server’s address and the request handler class.
3. Then call the handle_request() orserve_forever() method of the server object to process one or many requests.
4. Finally, call server_close() to close the socket.

基本的socketserver代码

 import socketserver

 class MyTCPHandler(socketserver.BaseRequestHandler):
     """
     The request handler class for our server.

     It is instantiated once per connection to the server, and must
     override the handle() method to implement communication to the
     client.
     """

     def handle(self):
         # self.request is the TCP socket connected to the client
         self.data = self.request.recv(1024).strip()
         print("{} wrote:".format(self.client_address[0]))
         print(self.data)
         # just send back the same data, but upper-cased
         self.request.sendall(self.data.upper())

 if __name__ == "__main__":
     HOST, PORT = "localhost", 9999

     # Create the server, binding to localhost on port 9999
     server = socketserver.TCPServer((HOST, PORT), MyTCPHandler)

     # Activate the server; this will keep running until you
     # interrupt the program with Ctrl-C
     server.serve_forever()

但你发现，上面的代码，依然不能同时处理多个连接，擦，那我搞这个干嘛？别急，不是不能处理多并发，如果你想，你还要启用多线程，多线程我们现在还没讲，但你大体知道，有了多线程，就能同时让cpu干多件事了就行先。

让你的socketserver并发起来， 必须选择使用以下一个多并发的类

class socketserver.ForkingTCPServer

class socketserver.ForkingUDPServer

class socketserver.ThreadingTCPServer

class socketserver.ThreadingUDPServer

so 只需要把下面这句

server = socketserver.TCPServer((HOST, PORT), MyTCPHandler)

换成下面这个，就可以多并发了，这样，客户端每连进一个来，服务器端就会分配一个新的线程来处理这个客户端的请求

server = socketserver.ThreadingTCPServer((HOST, PORT), MyTCPHandler)

class socketserver.BaseServer(server_address, RequestHandlerClass) 主要有以下方法

 class socketserver.BaseServer(server_address, RequestHandlerClass)
 This is the superclass of all Server objects in the module. It defines the interface, given below, but does not implement most of the methods, which is done in subclasses. The two parameters are stored in the respective server_address and RequestHandlerClass attributes.

 fileno()
 Return an integer file descriptor for the socket on which the server is listening. This function is most commonly passed to selectors, to allow monitoring multiple servers in the same process.

 handle_request()
 Process a single request. This function calls the following methods in order: get_request(), verify_request(), and process_request(). If the user-provided handle() method of the handler class raises an exception, the server’s handle_error() method will be called. If no request is received within timeout seconds, handle_timeout() will be called and handle_request() will return.

 serve_forever(poll_interval=0.5)
 Handle requests until an explicit shutdown() request. Poll for shutdown every poll_interval seconds. Ignores the timeout attribute. It also calls service_actions(), which may be used by a subclass or mixin to provide actions specific to a given service. For example, the ForkingMixIn class uses service_actions() to clean up zombie child processes.

 Changed in version 3.3: Added service_actions call to the serve_forever method.

 service_actions()
 This is called in the serve_forever() loop. This method can be overridden by subclasses or mixin classes to perform actions specific to a given service, such as cleanup actions.

 New in version 3.3.

 shutdown()
 Tell the serve_forever() loop to stop and wait until it does.

 server_close()
 Clean up the server. May be overridden.

 address_family
 The family of protocols to which the server’s socket belongs. Common examples are socket.AF_INET and socket.AF_UNIX.

 RequestHandlerClass
 The user-provided request handler class; an instance of this class is created for each request.

 server_address
 The address on which the server is listening. The format of addresses varies depending on the protocol family; see the documentation for the socket module for details. For Internet protocols, this is a tuple containing a string giving the address, and an integer port number: ('127.0.0.1', 80), for example.

 socket
 The socket object on which the server will listen for incoming requests.

 The server classes support the following class variables:

 allow_reuse_address
 Whether the server will allow the reuse of an address. This defaults to False, and can be set in subclasses to change the policy.

 request_queue_size
 The size of the request queue. If it takes a long time to process a single request, any requests that arrive while the server is busy are placed into a queue, up to request_queue_size requests. Once the queue is full, further requests from clients will get a “Connection denied” error. The default value is usually 5, but this can be overridden by subclasses.

 socket_type
 The type of socket used by the server; socket.SOCK_STREAM and socket.SOCK_DGRAM are two common values.

 timeout
 Timeout duration, measured in seconds, or None if no timeout is desired. If handle_request() receives no incoming requests within the timeout period, the handle_timeout() method is called.

 There are various server methods that can be overridden by subclasses of base server classes like TCPServer; these methods aren’t useful to external users of the server object.

 finish_request()
 Actually processes the request by instantiating RequestHandlerClass and calling its handle() method.

 get_request()
 Must accept a request from the socket, and return a 2-tuple containing the new socket object to be used to communicate with the client, and the client’s address.

 handle_error(request, client_address)
 This function is called if the handle() method of a RequestHandlerClass instance raises an exception. The default action is to print the traceback to standard output and continue handling further requests.

 handle_timeout()
 This function is called when the timeout attribute has been set to a value other than None and the timeout period has passed with no requests being received. The default action for forking servers is to collect the status of any child processes that have exited, while in threading servers this method does nothing.

 process_request(request, client_address)
 Calls finish_request() to create an instance of the RequestHandlerClass. If desired, this function can create a new process or thread to handle the request; the ForkingMixIn and ThreadingMixIn classes do this.

 server_activate()
 Called by the server’s constructor to activate the server. The default behavior for a TCP server just invokes listen() on the server’s socket. May be overridden.

 server_bind()
 Called by the server’s constructor to bind the socket to the desired address. May be overridden.

 verify_request(request, client_address)
 Must return a Boolean value; if the value is True, the request will be processed, and if it’s False, the request will be denied. This function can be overridden to implement access controls for a server. The default implementation always returns True.