Python continuous TCP connection

Question

I'm new to socket programming and trying to get back into python. I wanted to write a simple TCP program which will continuously maintain a connection until some end state is reached, in this case "close" is sent by the client.

This works fine for the first iteration, but it freezes on the second thing I send and I'm not sure why. Could someone please explain why my program freezes or how better implement this?

TCPServer.py

from socket import *
serverPort = 12000
serverSocket = socket(AF_INET, SOCK_STREAM)
serverSocket.setsockopt(SOL_SOCKET, SO_REUSEADDR, 1)
serverSocket.bind(('', serverPort))
serverSocket.listen(1)
print('The server is ready to recieve')

while True:
    connectionSocket, addr = serverSocket.accept()
    sentence = connectionSocket.recv(1024).decode()
    if(sentence == "close"):
        connectionSocket.close()
    capSentence = sentence.upper()
    connectionSocket.send(capSentence.encode())

TCPClient.py

from socket import *
serverName = 'localhost'
serverPort = 12000
clientSocket = socket(AF_INET, SOCK_STREAM)
clientSocket.connect((serverName, serverPort))
while(1):
    sentence = raw_input('input sentence: ')
    if(sentence == "close"):
        break
    clientSocket.send(sentence.encode())
    retSentence = clientSocket.recv(1024)
    print('From server; ', retSentence.decode())

clientSocket.close()

Answer 1

There are multiple problems with the server. I'll try to point out what I see.

It freezes on the second thing you send because the server never attempts to read from the client socket a second time if the received data is not equal to "close". You need another loop to accomplish this, such as:

while True:
    connectionSocket, addr = serverSocket.accept()

    while True:
        sentence = connectionSocket.recv(1024).decode()
        if sentence == "close": break
        capSentence = sentence.upper()
        connectionSocket.send(capSentence.encode())

    connectionSocket.close()

Additionally, note that the server runs through the loops synchronously, and so it cannot accept a second connection until the first one is closed.

Finally, you are missing error handling code. The whole server will crash if a client prematurely disconnects.

---

If your goal is to have the server only handle a single connection and then terminate, the simplest solution is to move the accept() call outside of the main loop (otherwise you wait for a new connection on each iteration).

connectionSocket, addr = serverSocket.accept()

# Destroy the server socket; we don't need it anymore since we are not
# accepting any connections beyond this point.
serverSocket.close()

while True:
    sentence = connectionSocket.recv(1024).decode()
    if(sentence == "close"):
        connectionSocket.close()
        break
    capSentence = sentence.upper()
    connectionSocket.send(capSentence.encode())