simpdownloader.py

import os, sys
import traceback

import mechanize

KILOBYTE = 1024
MEGABYTE = KILOBYTE * KILOBYTE
BUFFER = MEGABYTE / 2

class Downloader(object):
    def __init__(self):
        self._browser = None

    def browser(self):
        if not self._browser:
            self._browser = mechanize.Browser()
            
        return self._browser
            
    def open(self, url, retry=False, timeout=5):
        while True:
            try:
                self.response = self.browser().open(url, timeout=timeout)
            except Exception as e:
            
                if retry: 
                    continue
                else:
                    return None
                    
            return self.response
    
    def download(self, outfile=None):
        totalbytes = 0
        
        if not outfile:
            outfile = os.tmpfile()
        
        while True:
            # Read bit by bit, to keep memory usage down.
            data = self.response.read(BUFFER)
            
            if not data:
                done = True
                break
                
            outfile.write(data)
            totalbytes += len(data)
            print "Got %d bytes, %d total." % (len(data), totalbytes)
                
        outfile.seek(0)
        return outfile

test.py

from simpdownloader import Downloader, BUFFER

def main():
    dl = Downloader()
    
    filename = "ubuntu.iso"
    url = "http://www.ubuntu.com/start-download?distro=desktop&bits=32&release=latest"
    
    try:
        dl.open(url, retry=True)
        tmpfile = dl.download()
    except Exception as e:
        print "Error downloading '%s': %s" % (outfile, e.message)
    else:
        if tmpfile:
            with open(filename, "wb") as newfile:
                while True:
                    data = tmpfile.read(BUFFER)
                    if not data:
                        break
                    
                    newfile.write(data)
                    
            tmpfile.close()
            
if __name__ == "__main__":
    main()