> I'm good with None/True, but that would imply that for posix rename
> we'll need to implement the overwrite=False option...which would be a
> nice thing (the shell mv command has -i for that).

My point was rather to forbid False as a value (on all OSes) :)

> I think a warning would be good, because a unix programmer will assume
> rename will work the same on windows as it does on posix, and vice
> versa for a windows programmer.

It is already documented. I don't think we want to add a warning for
every documented peculiar piece of behaviour.