Documentation of tempfile.gettempdir() <https://docs.python.org/3/library/tempfile.html#tempfile.gettempdir> does not mention it only supports existing directories.

This led to this question on Stackoverflow: http://stackoverflow.com/q/37229398/539465

I don't understand. The documentation clearly states:

"Python searches a standard list of directories to find one which the calling user can create files in." How do you expect to be able to write files to a directory that doesn't exist?

The directory could be created

No, it shouldn't.

The design is that different systems use different locations for the default tempdir, and we look down a list of ones we know about until we find one that exists and can be used.  Likewise, different shells use different environment variables, so we look at a sequence of those as well.

It is arguable whether the fact that an environment variable that points to an invalid directory is ignored is a feature or a bug, but at this point tempfile has behaved that was for so long that we must class it as a feature.

In any case, I would find it objectionable for a program to create a temporary directory for me unasked.  It would be much more likely that I misspelled the directory when I set the environment variable.

I know, that's why I did not ask to change the code. Only the doc to make that behavior more explicit.

As Steve pointed out, the documentation is explicit: "to *find* one the user can craete files in".  You can't find something that doesn't yet exist.