I ran into an error today related to the use of support.TESTFN throughout the regression test suite. In my Windows tests, test_base64 passed, but left a file (named by support.TESTFN) lying around: 'test_base64' left behind file '@test_3532_tmp' Much later in the run, a set of unrelated tests started failing, all apparently because of the left-behind file - for example, test_mailbox.setUp failed while trying to make a directory named by support.TESTFN: File "c:\Users\Vinay\Projects\pythonv\lib\mailbox.py", line 270, in __init__ os.mkdir(self._path, 0o700) PermissionError: [Error 5] Access is denied: 'c:\\Users\\Vinay\\Projects\\pythonv\\build\\test_python_3532\\@test_3532_tmp' Sorry if this has come up before, but why do we couple the tests in this way, so that failure to clean up in one test causes drive-by failures in other, unrelated tests? Regards, Vinay Sajip