> def test_chdir():
>     with tempfile.TemporaryDirectory() as tempdir:
>         old = os.getcwd()
>         os.chdir(tempdir)
>         with open(os.path.join(tempdir, "delme")) as fout:
>             fout.write("Hello")
>         os.chdir(old)

The open() call in test_chdir() fails before os.chdir(old) executes. You would need to call os.chdir(old) in a `finally` block.

But the cleanup routine could still fail due to a sharing violation from some other process (likely a child process) setting the directory or a child directory as the working directory. Also, a sharing violation when trying to delete a regular file in the tree causes the cleanup routine to fail with NotADirectoryError because the onerror function calls rmtree() if unlink() fails with a PermissionError.