By default, TCP sockets are placed in a blocking mode. This means the control is not returned to your program until some specific operation is complete. If you call the connect() API, the connection blocks your program until the operation is complete. On many occasions, you don't want to keep your program waiting forever, either for a response from the server or for any error to stop the operation. For example, when you write a web browser client that connects to a web server, you should consider a stop functionality that can cancel the connection process in the middle of this operation. This can be achieved by placing the socket in the non-blocking mode.

Let us see what options are available under Python. In Python, a socket can be placed in the blocking or non-blocking mode. In the non-blocking mode, if any call to API, for example, send() or recv(), encounters any problem, an error will be raised. However, in the blocking mode, this will not stop the operation. We can create a normal TCP socket and experiment with both the blocking and non-blocking operations.

To manipulate the socket's blocking nature, we should create a socket object first.

We can then call setblocking(1) to set up blocking or setblocking(0) to unset blocking. Finally, we bind the socket to a specific port and listen for incoming connections.

Listing 1.9 shows how the socket changes to blocking or non-blocking mode as follows:

#!/usr/bin/env python 
# Python Network Programming Cookbook, Second Edition -- Chapter - 1 
# This program is optimized for Python 2.7.12 and Python 3.5.2. 
# It may run on any other version with/without modifications. 
 
import socket 
 
def test_socket_modes(): 
    s = socket.socket(socket.AF_INET, 
                       socket.SOCK_STREAM) 
    s.setblocking(1) 
    s.settimeout(0.5) 
    s.bind(("127.0.0.1", 0)) 
     
    socket_address = s.getsockname() 
    print ("Trivial Server launched on 
             socket: %s" %str(socket_address)) 
    while(1): 
        s.listen(1) 
   
if __name__ == '__main__': 
    test_socket_modes() 

If you run this recipe, it will launch a trivial server that has the blocking mode enabled as shown in the following command:

$ python 1_9_socket_modes.py 
Trivial Server launched on 
socket: ('127.0.0.1', 51410)
  

In this recipe, we enable blocking on a socket by setting the value 1 in the setblocking() method. Similarly, you can unset the value 0 in this method to make it non-blocking.

This feature will be reused in some later recipes, where its real purpose will be elaborated.