> Developers may think that the file descriptor is closed when the integer
> object gets out of scope.

I'm not concerned about that. When they use os.open, they will typically
understand what a file handle is, and how it relates to

> I propose the addition of opendir() for the purpose of the *at()
> functions. The opendir function should return a wrapper object around
> the DIR* pointer returned by opendir(). A function fileno() exposed the
> file descriptor of the DIR pointer via dirfd().

-1. This is exactly the second-guessing kind of thing that the POSIX
module should avoid. openat(2) doesn't expect DIR objects, and neither
should posix.openat.

If desired, a layer can be added on top of this that makes it more
"safe", e.g. in the os module; such a layer should then try to make it
cross-platform before trying to make it safe.