As you said, this race condition can happen even if you copy the file in
some temp dir, then move them to python. That would reduce the window,
but not remove it.

The best way to avoid it is to stop any running Python process when 
updates occurs I guess, or work on some kind of protected import statement. 

Now I am not sure about potential breakages during installation at
Distutils' level since it just copies files to site-packages only once
everything is built. The only breakage I have in mind is when there's
some kind of I/O error when copying files on the disk.

So I agree that it would be better to save any file that is about to be
overriden when the install command starts to copy files, so it can
rollback if a problem occurs.