Could you add a docstring to this function to explain its purpose? It's pretty obvious what it does, but not necessarily why it's needed.
As far as I can tell, this is simply a combination of a buffered reader and a writer, to avoid reading large files into memory entierely. I'm wondering if the same couldn't be achieved with a combination of [`io.BufferedReader`](http://docs.python.org/2/library/io.html#io.BufferedReader) and friends.