Let us download the Python 2.7 code from http://www.python.org. A resume_download() file will resume any unfinished download of that file.

Listing 4.9 explains resume downloading as follows:

#!/usr/bin/env python 
# Python Network Programming Cookbook -- Chapter - 4 
# This program requires Python 3.5.2 or any later version 
# It may run on any other version with/without modifications. 
# 
# Follow the comments inline to make it run on Python 2.7.x. 
 
import urllib.request, urllib.parse, urllib.error 
# Comment out the above line and uncomment the below for Python 2.7.x. 
#import urllib 
 
import os 
 
TARGET_URL = 'http://python.org/ftp/python/2.7.4/' 
TARGET_FILE = 'Python-2.7.4.tgz' 
 
class CustomURLOpener(urllib.request.FancyURLopener): 
# Comment out the above line and uncomment the below for Python 2.7.x. 
#class CustomURLOpener(urllib.FancyURLopener): 
    """Override FancyURLopener to skip error 206 (when a 
       partial file is being sent) 
    """ 
    def http_error_206(self, url, fp, errcode, errmsg, headers, data=None): 
        pass 
 
def resume_download(): 
   file_exists = False 
   CustomURLClass = CustomURLOpener() 
   if os.path.exists(TARGET_FILE): 
         out_file = open(TARGET_FILE,"ab") 
         file_exists = os.path.getsize(TARGET_FILE) 
         #If the file exists, then only download the unfinished part 
         CustomURLClass.addheader("range","bytes=%s-" % (file_exists)) 
   else: 
         out_file = open(TARGET_FILE,"wb") 
 
   web_page = CustomURLClass.open(TARGET_URL + TARGET_FILE) 
 
   #Check if last download was OK 
   if int(web_page.headers['Content-Length']) == file_exists: 
         loop = 0 
         print ("File already downloaded!") 
 
   byte_count = 0 
   while True: 
         data = web_page.read(8192) 
         if not data: 
               break 
         out_file.write(data) 
         byte_count = byte_count + len(data) 
 
   web_page.close() 
   out_file.close() 
 
   for k,v in list(web_page.headers.items()): 
    # Comment out the above line and uncomment the below for Python 2.7.x. 
   #for k,v in web_page.headers.items(): 
         print (k, "=",v) 
   print ("File copied", byte_count, "bytes from", web_page.url) 
 
if __name__ == '__main__': 
   resume_download() 
 

Executing this script will result in the following output:

$   python 4_9_http_fail_over_client.py
content-length = 14489063
content-encoding = x-gzip
accept-ranges = bytes
connection = close
server = Apache/2.2.16 (Debian)
last-modified = Sat, 06 Apr 2013 14:16:10 GMT
content-range = bytes 0-14489062/14489063
etag = "1748016-dd15e7-4d9b1d8685e80"
date = Tue, 07 May 2013 12:51:31 GMT
content-type = application/x-tar
File copied 14489063 bytes from http://python.org/ftp/python/2.7.4/Python-2.7.4.tgz