w9xpopen is currently used on NT. The patch to use it on NT was checked in by bquinlan in August of 2001 (http://mail.python.org/pipermail/patches/2001-August/005719.html). He claims that it is necessary in NT, even though (a) the cited knowledge base article explicitly states that it is not necessary on NT, and (b) the knowledge base article has now been deleted from Microsoft's web site, indicating that they consider it no longer relevant (they have deleted all Win9x-specific documentation, but Win2K-specific documentation is still there).

I just don't believe that the problem solved by w9xpopen has ever existed in any version of NT. There is no credible evidence for it. There are any number of other reasons why introducing an intermediate process might have hidden some unrelated bug or otherwise resolved the problem the Win9x->Win2K upgraders were having a decade ago. I think that the use of w9xpopen in NT is a bug, not an obsolete feature, and there's no reason it couldn't be gone in 3.2.1.

Also, I suppose it doesn't matter any more, but the logic for deciding when to run w9xpopen should be (target executable is 16-bit), which can be determined by reading the file header. Right now the test is (shell is True and (running on win9x or the command processor is named "command.com")). Every part of this test is deficient. Python programs can spawn 16-bit processes (including the shell itself) without using shell=True. Not every win9x shell is 16-bit; 32-bit shells like cmd.exe work fine. And there are 16-bit shells not named "command.com", such as 4DOS.