I've also run into this bug on Windows. In my case, the tempdir path includes directories on a network share, which I lack write access permissions to. Python tries to generate a *lot* of files, and never figures out it should move on to another directory. The attached patch for tempdir.py resolves my issue.

In _get_default_tempdir() and _mkstemp_inner(), you want to know if the filename you tried to create already exists as a directory, not whether the parent directory is a directory – that's handled in _get_default_tempdir().

In mkdtemp(), attempting to create a directory with the same name as an existing directory does not throw a PermissionError, so the code is superfluous.