# Simple socket server-client application in Python 3

In this tutorial I am going to give another example of simple socket server. This is very handy when you need to have running daemon on background.

My scenario was as follows. Initialize the master server process which will wait for client's connection. Wait for him to send some input. With this input, do some work, process it and then return the result to the client. All this without needing reinitializing the whole server process (because starting it take very long time). The server process should be able to accept several connections at once - it should start a separate thread for each client.

Our server and client will communicate on localhost, hence 127.0.0.1 and port 12345. But of course, you could choose your own. In function start_server we initialize the socket and prepare server for accepting several clients at once (otherwise the processes would need to wait until the previous client ends the connection). It is done by calling Thread with the function client_thread. In this function we call do_some_stuffs_with_input which can be any function you wish.

The code for server is below.

# server.py

def do_some_stuffs_with_input(input_string):
"""
This is where all the processing happens.

Let's just read the string backwards
"""

print("Processing that nasty input!")
return input_string[::-1]

def client_thread(conn, ip, port, MAX_BUFFER_SIZE = 4096):

# the input is in bytes, so decode it
input_from_client_bytes = conn.recv(MAX_BUFFER_SIZE)

# MAX_BUFFER_SIZE is how big the message can be
# this is test if it's sufficiently big
import sys
siz = sys.getsizeof(input_from_client_bytes)
if  siz >= MAX_BUFFER_SIZE:
print("The length of input is probably too long: {}".format(siz))

# decode input and strip the end of line
input_from_client = input_from_client_bytes.decode("utf8").rstrip()

res = do_some_stuffs_with_input(input_from_client)
print("Result of processing {} is: {}".format(input_from_client, res))

vysl = res.encode("utf8")  # encode the result string
conn.sendall(vysl)  # send it to client
conn.close()  # close connection
print('Connection ' + ip + ':' + port + " ended")

def start_server():

import socket
soc = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
# this is for easy starting/killing the app
print('Socket created')

try:
soc.bind(("127.0.0.1", 12345))
print('Socket bind complete')
except socket.error as msg:
import sys
print('Bind failed. Error : ' + str(sys.exc_info()))
sys.exit()

#Start listening on socket
soc.listen(10)
print('Socket now listening')

# this will make an infinite loop needed for
# not reseting server for every client
while True:
print('Accepting connection from ' + ip + ':' + port)
try:
except:
print("Terible error!")
import traceback
traceback.print_exc()
soc.close()

start_server()


Code for client is very simple:

# client.py

import socket

soc = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
soc.connect(("127.0.0.1", 12345))

clients_input = input("What you want to proceed my dear client?\n")
soc.send(clients_input.encode("utf8")) # we must encode the string to bytes
result_bytes = soc.recv(4096) # the number means how the response can be in bytes
result_string = result_bytes.decode("utf8") # the return will be in bytes, so decode

print("Result from server is {}".format(result_string))


All you need to do now is to run the server script in one window by python server.py and then run the client by python client.py in some another. It should ask you for input and after pressing the enter the server should return the output.

# Periodically push date to server

It might be handy to be able to push data periodically without needing to open connection again and again. Here is an example what I did when I want to push row by row two column table.

# client

import socket

soc = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
soc.connect(("127.0.0.1", 12345))

import random
data = zip([random.randint(1,1000) for i in range(1000)],
[random.randint(1,1000) for i in range(1000)])

for x, y in data:
# send x and y separated by tab
data = "{}\t{}".format(x,y)
soc.sendall(data.encode("utf8"))

# wait for response from server, so we know
# that server has enough time to process the
# data. Without this it can make problems

if soc.recv(4096).decode("utf8") == "-":
pass

# end connection by sending this string
soc.send(b'--ENDOFDATA--')


and the server side is as follows (start_server function is same as above):

# server
def rec_data(conn, MAX_BUFFER_SIZE):
input_from_client_bytes = conn.recv(MAX_BUFFER_SIZE)

import sys
siz = sys.getsizeof(input_from_client_bytes)
if  siz >= MAX_BUFFER_SIZE:
print("The length of input is probably too long: {}".format(siz))

input_from_client = input_from_client_bytes.decode("utf8").rstrip()

return input_from_client

def client_thread(conn, ip, port, MAX_BUFFER_SIZE = 88888):

# read lines periodically without ending connection
still_listen = True
while still_listen:
input_from_client = rec_data(conn, MAX_BUFFER_SIZE)

# if you receive this, end the connection
if "--ENDOFDATA--" in input_from_client:
print('--ENDOFDATA--')
conn.close()
print('Connection ' + ip + ':' + port + " ended")
still_listen = False
else:
splin = input_from_client.split('\t')

print("{}, {}".format(splin[0], splin[1]))

# tell client that we can accept another data processing
conn.sendall("-".encode("utf8"))