Thanks for the clarifications and support.
Ah, the wording in the PEP says "Linux, Windows, OS X". Superficially, that said "everywhere" to me. It might be worth calling out specifically some examples where it's not available without an extra system call, just to make the point explicit.
Good call. I'll update the wording in the PEP here and try to call out specific examples of where is_dir() could call os.stat(). Hard-core POSIX people, do you know when readdir() d_type will be DT_UNKNOWN on (for example) Linux or OS X? I suspect this can happen on certain network filesystems, but I'm not sure.
PS I'd suggest adding a "Rejected proposals" section to the PEP which mentions the race condition issue and points to this discussion as an indication that people didn't seem to see it as a problem.
Definitely agreed. I'll add this, and clarify various other issues in the PEP, and then repost. -Ben