It is possible to do partial download using the range header, the following will request a selected range of bytes:
req = urllib2.Request('http://www.python.org/')
req.headers['Range'] = 'bytes=%s-%s' % (start, end)
f = urllib2.urlopen(req)
For example:
>>> req = urllib2.Request('http://www.python.org/')
>>> req.headers['Range'] = 'bytes=%s-%s' % (100, 150)
>>> f = urllib2.urlopen(req)
>>> f.read()
'l1-transitional.dtd">
<html xmlns="http://www.w3.'
Using this header you can resume partial downloads. In your case all you have to do is to keep track of already downloaded size and request a new range.
Keep in mind that the server need to accept this header for this to work.
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