Partial download with ftplib and retrbinary

[fepeacock at googlemail.com]

I am breaking/interrupting my connection with the ftp server at
present when doing a partial download of a file. I have a callback
with retrbinary that raises an exception and ends the download. The
problem is that the server is not notified and hangs. I cannot send
any further commands and need to relogin to download further. Is there
a way to still continue downloading without having to login again.

My retrbinary function is:

ftp.retrbinary('RETR '+file, handleDownload,1,bound[0])

where bound[0] is an integer specifying the start byte of the
download.

My callback is:

def handleDownload(block):
    global count,localfile,number
    localfile.write(block)
    if count==number:
            raise(Exception)
    count=count+1

where number specifies the number of bytes to download.

Help would be gratefully received.