Python Socket Programming

How to build socket server and socket client by python?

The first think to decide is which protocol to use, TCP or UDP?

The main difference is that TCP is connection-oriented while UDP is connectionless.

In detail, the server or the client must maintain a connection if they use TCP.

How is this reflected in socket programming?

For UDP

For example, if we want to send a message using UDP

import socket
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
# s.bind('0.0.0.0', 2333)
address = ('123.123.123.123', 2333)
s.sendto(b'hi~', address)

Only the target address is required.

And the receiver (server) only need to specify its address using bind as a unique identifier on the internet so that the message sender could know where it is.

import socket
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.bind('0.0.0.0', 2333)
bufsize = 8192
while 1:
    message, address = s.recvfrom(bufsize)
    print(message)
    s.sendto(b'hello', address)

The main characteristic of UDP is that every sending of message is isolated and context-free.

Just sendto(message, address) no matter the sender is the server or the client, the boundary between the server and the client is vague and the distinguishment of server and client is unimportant.

For TCP

But when it comes to TCP, things becomes difficult.

For the message sender:

import socket
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
# s.bind('0.0.0.0', 2333)
address = ('123.123.123.123', 2333)
s.connect(address)
bufsize = 8192
while 1:
    s.send(b'hi~')
    msg = s.recv(bufsize)
    print(msg)
s.close()

Before sending the message, it should do connect first and close the connection after communication.

The target address would be sepcify only when the connection is created.

For the message receiver (server):

import socket
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.bind('0.0.0.0', 2333)
max_conn = 5
s.listen(max_conn)
while 1:
    conn, address = s.accept()
    bufsize = 1024
    while 1:
        try:
            msg = conn.recv(bufsize)
            print(msg)
            conn.send(b'hello')
        except:
            print('connection break')
            break
    conn.close()

The server should wait for connections and maintain them until ending.

In fact, a connection is also a socket.socket object (a file discriptor).

Summary

for UDP:

1. init
2. bind address if is receiver (server)
3. sendto or recvfrom

for TCP client:

1. init
2. connect to a server
3. send and recv messages
4. close the connection

for TCP server:

1. init
2. bind an address
3. listen continuously
4. accept a connection
5. send and recv messages
6. close the connection