Python socket.recv exception

Question

I'm working on a very simple python socket server. I use non-blocking sockets. The server and client are running on windows 7 x64 with python 2.7.3. Here is the code where I receive data from the client :

def listenToSockets(self):

    while True:

        changed_sockets = self.currentSockets

        ready_to_read, ready_to_write, in_error = select.select(changed_sockets,[],[])

        for s in ready_to_read:
            # if its the server master socket someone is connecting
            if s == self.socket:
                (client_socket, address) = s.accept()
                print "putting " + address[0] + " onto connections\n";
                client_socket.setblocking(0)

                self.currentSockets.append(client_socket)
                print "current client count : " + str(len(self.currentSockets) - 1)

            else:

                data = ''
                try:

                    while True:
                        part = s.recv(4096)
                        if part != '':
                            data = data + part
                        elif part == '':
                            break


                except socket.error, (value,message): 
                    print 'socket.error - ' + message


                if data != '':
                    print "server received "+data
                else:
                    print "Disconnected "+str(s.getsockname())

                    self.currentSockets.remove(s)
                    s.close()

And here is the client sending some data over and over again :

#client example
import socket
import time

client_socket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
client_socket.connect(('192.168.0.41', 9999))


while 1:
    client_socket.send("test")
    time.sleep(2) 

I see the server receiving the message "test" over and over again. But before it prints out what it received I get the following error message.

socket.error - A non-blocking socket operation could not be completed immediately.

Obviously an exception is thrown at part = s.recv(4096) but why?

Solution

That's precisely what a nonblocking socket is supposed to do.

• Read the available data, if any
• If nothing is available, raise an error rather than blocking

So you're getting an exception every time you try to receive and there's no data available.