truncating a file from the top down

[rbt]

Hi guys,
I need to truncate a file from the top down. I imagine doing something 
like this:

if os.stat says the file is too big:
    read the file
    trim = only keep the last 2008 bytes (This is where I get stuck)
    write trim back out to the original file
Would someone demonstrate the *best* most efficient way of doing this?
Thanks,
rbt
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