Python - Socket - client, server

# --- SERVER ---

#!/usr/bin/env python3

#
# https://docs.python.org/3.5/library/socket.html
#

import socket
#import time

# - constants -

HOST = ''   # local address IP (not external address IP)

            # '0.0.0.0' or '' - conection on all NICs (Network Interface Card),
            # '127.0.0.1' or 'localhost' - local conection only (can't connect from remote computer)
            # 'local_IP' - connection only on one NIC which has this IP

PORT = 8000 # local port (not external port)

# - create socket -

print('[DEBUG] create socket')
#s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s = socket.socket() # default value is (socket.AF_INET, socket.SOCK_STREAM)
                    # so you don't have to use it in socket()

# - options -

print('[DEBUG] set options')
# solution for "[Error 89] Address already in use". Use before bind()
s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)

# - assign socket to local IP (local NIC)  -

print('[DEBUG] bind:', HOST, PORT)
s.bind((HOST, PORT)) # one tuple (HOST, PORT), not two arguments

# - set size of queue -

print('[DEBUG] listen')
s.listen(1) # number of clients waiting in queue for "accept".
            # If queue is full then client can't connect.

try:
    # - for test only - you don't need it -

    # client can wait for accept()
    # but can't wait for bind() and listen()

    #print('[DEBUG] sleep ... for test only (normally you don\'t need it)')
    #time.sleep(5)  # waiting for test only (normally you don\'t need it)

    # - accecpt client -

    # accept client and create new socket `conn` (with different port) for this client only
    # and server will can use `s` to accept other clients (if you will use threading)

    print('[DEBUG] accept ... waiting')
    conn, addr = s.accept()
    print('[DEBUG] addr:', addr)

    # - recevier/send data -

    # if client first `send()` and next `recv()`
    # then server have to first `recv`() and next `send()`

    # if both will `recv()` at the same time then all will hang
    # because both will wait for data and nobody will `send()`

    # if don't use native characters
    # then you can use 'ascii' instead of 'utf-8'

    # receiving

    print('[DEBUG] receive')
    data = conn.recv(1024)
    text = data.decode('utf-8') # decode bytes to string
    print(text)

    # sending

    print('[DEBUG] send')
    text = 'Goodbye World of Sockets in Python'
    data = text.encode('utf-8') # encode string to bytes
    conn.send(data)
    print(text)

except Exception as e:
    print(e)

# - close all sockets -

# alway first close `conn`, next close `s`
print('[DEBUG] close socket(s)')
conn.close()
s.close()




# --- CLIENT ---

#!/usr/bin/env python3

#
# https://docs.python.org/3.5/library/socket.html
#

import socket

# - constants -

HOST = ''    # (local or external) address IP of remote server
PORT = 8000  # (local or external) port of remote server

# server can have local address IP - used only in local network
# or external address IP - used in internet on external router
# (and router redirects data to internal address IP)

# - create socket -

print('[DEBUG] create socket')
#s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s = socket.socket() # default value is (socket.AF_INET, socket.SOCK_STREAM)
                    # so you don't have to use it in socket()

# - connect to server -

print('[DEBUG] connect:', HOST, PORT)
s.connect((HOST, PORT)) # one tuple (HOST, PORT), not two arguments

# - send data -

# if don't use native characters
# then you can use 'ascii' instead of 'utf-8'

print('[DEBUG] send')
text = "Hello World of Sockets in Python"
data = text.encode('utf-8') # encode string to bytes
s.send(data)
print(text)

# - receive data -

# if don't use native characters
# then you can use 'ascii' instead of 'utf-8'

print('[DEBUG] receive')
data = s.recv(1024)
text = data.decode('utf-8') # decode bytes to string
print(text)

# - close socket -

print('[DEBUG] close socket')
s.close()