> So is this a cosmetic issue or a functional issue? 

It's a cosmetic issue.

> Also, even if it could figure that out, how would it know whether
> a particular filename "stringification" with os.path.join() was 
> intended for display to the user or to be passed to a Windows API?

The Windows API's (at least every one I've ever called) take either
slashes or backslashes, so it wouldn't matter.

I'm +0 on this request, if we could reliably figure out which separator
we wanted to use based on the shell (or maybe an environment variable).
It would be one more way to let me forget I'm using Windows instead of Unix.

My use case is mostly copying path's that have been print()'d from
within Python, then pasting them into a cygwin bash shell. All of the
backslashes need to be manually escaped (or the whole string quoted). Of
course this doesn't help with other characters that also need escaping
(like spaces). That and the fragile nature of the "which shell am I
running" check are why I'm +0.