import argparse,socket

 from datetime import datetime

 MAX_BYTES = 65535

 def server(port):

     sock = socket.socket(socket.AF_INET,socket.SOCK_DGRAM) #协议族AF_INET以及数据报类型SOCK_DGRAM.SOCK_DGRAM表示在IP网络上使用UDP协议

                                     #要注意的是数据报datagram而不是数据包packet,用了表示应用层数据块传输的官方术语

     sock.bind(('127.0.0.1',port))    #绑定一个UDP网络地址：一个二元组，包含一个IP地址和一个整形的UDP端口号

                                              #端口号要大于1023，否则要管理员权限

     print('Listening at {}'.format(sock.getsockname())) #查询套接字当前绑定的IP地址和端口号

     while True:                                  #会进入一个循环不断运行recvfromO

         data,adress = sock.recvfrom(MAX_BYTES)  #最长可接收65535字节的信息，也是UDP数据报可包含的最大长度，没有接收到永远等待

                                                  #一旦接收到数据报，返回两个值。1 发送该数据报的客户端地址 2 以字节表示数据报的内容

         text = data.decode('ascii')

         print('The client at {} says {!r}'.format(adress,text))

         text = 'Your data was {} bytes long'.format(len(data))

         data = text.encode('ascii')

         sock.sendto(data,adress)

 def client(port):

     sock = socket.socket(socket.AF_INET,socket.SOCK_DGRAM)

     text = 'The time is {}'.format(datetime.now())

     data = text.encode('ascii')

     sock.sendto(data,('127.0.0.1',port))  #两个信息：要发送的信息和目标地址。这是向服务器发送数据报的唯一调用

     print('The OS assigned me the adress {}'.format(sock.getsockname()))

     data,adress = sock.recvfrom(MAX_BYTES)

     text = data.decode('asicc')

     print('The serve {} replied {!r}'.format(adress,text))

     if __name__ == '__main__':

         choices = {'client':client,'server':server}

         parser = argparse.ArgmentParser(description='Send and receive UDP locally')

         parser.add_argument('role',choices = choices ,help = 'which role to play')

         parser.add_argument('-p',metavar = 'PORT',type = int ,default = 1060,

                             help = 'UDP port (default 1060)')

         args = parser.parse_args()

         function = choices[args.role]

         function(args.p)