#!/usr/bin/env python
# Python Network Programming Cookbook -- Chapter - 4
# This program is optimized for Python 2.7.# It may run on any other version with/without modifications.

import urllib, os
TARGET_URL = 'http://python.org/ftp/python/2.7.4/'
TARGET_FILE = 'Python-2.7.4.tgz'

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("Download range","bytes=%s-" % 
(file_exists))
  else:
    out_file = open(TARGET_FILE,"wb")

  web_page = CustomURLClass.open(TARGET_URL + TARGET_FILE)

  #If the file exists, but we already have the whole thing, don't 
download again
  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 web_page.headers.items():
    print k, "=",v
  print "File copied", byte_count, "bytes from", web_page.url

if __name__ == '__main__':
  resume_download()

$   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