Let us manipulate the default socket buffer size using a socket object's setsockopt() method.

First, define two constants: SEND_BUF_SIZE/RECV_BUF_SIZE and then wrap a socket instance's call to the setsockopt() method in a function. It is also a good idea to check the value of the buffer size before modifying it. Note that we need to set up the send and receive buffer size separately.

Listing 1.8 shows how to modify socket send/receive buffer sizes as follows:

#!/usr/bin/env python
# Python Network Programming Cookbook -- Chapter – 1
# This program is optimized for Python 2.7. It may run on any
# other Python version with/without modifications

import socket

SEND_BUF_SIZE = 4096
RECV_BUF_SIZE = 4096

def modify_buff_size():
    sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM )
    
    # Get the size of the socket's send buffer
    bufsize = sock.getsockopt(socket.SOL_SOCKET, socket.SO_SNDBUF)
    print "Buffer size [Before]:%d" %bufsize
    
    sock.setsockopt(socket.SOL_TCP, socket.TCP_NODELAY, 1)
    sock.setsockopt(
            socket.SOL_SOCKET,
            socket.SO_SNDBUF,
            SEND_BUF_SIZE)
    sock.setsockopt(
            socket.SOL_SOCKET,
            socket.SO_RCVBUF,
            RECV_BUF_SIZE)
    bufsize = sock.getsockopt(socket.SOL_SOCKET, socket.SO_SNDBUF)
    print "Buffer size [After]:%d" %bufsize

if __name__ == '__main__':
    modify_buff_size()

If you run the preceding script, it will show the changes in the socket's buffer size. The following output may be different on your machine depending on your operating system's local settings:

$ python 1_8_modify_buff_size.py 
Buffer size [Before]:16384
Buffer size [After]:8192

You can call the getsockopt() and setsockopt() methods on a socket object to retrieve and modify the socket object's properties respectively. The setsockopt() method takes three arguments: level, optname, and value. Here, optname takes the option name and value is the corresponding value of that option. For the first argument, the needed symbolic constants can be found in the socket module (SO_*etc.).