[Python-Dev] Tests failing on Windows with TESTFN

Brett Cannon brett at python.org
Thu Jul 26 13:11:57 EDT 2018

On Wed, 25 Jul 2018 at 15:16 Tim Golden <mail at timgolden.me.uk> wrote:

> I'm just easing back into core development work by trying to get a
> stable testing environment for Python development on Windows.
> One problem is that certain tests use support.TESTFN (a local directory
> constructed from the pid) for output files etc. However this can cause
> issues on Windows when recreating the folders / files for multiple
> tests, especially when running in parallel.
> Here's an example on my laptop deliberately running 3 tests with -j0
> which I know will generate an error about one time in three:
> C:\work-in-progress\cpython>python -mtest -j0 test_urllib2 test_bz2
> test_importlib
> Running Debug|Win32 interpreter...
> Run tests in parallel using 6 child processes
> 0:00:23 [1/3/1] test_urllib2 failed
> test test_urllib2 failed -- Traceback (most recent call last):
>    File "C:\work-in-progress\cpython\lib\test\test_urllib2.py", line
> 821, in test_file
>      f = open(TESTFN, "wb")
> PermissionError: [Errno 13] Permission denied: '@test_15564_tmp'
> Although these errors are both intermittent and fairly easily spotted,
> the effect is that I rarely get a clean test run when I'm applying a patch.
> I started to address this years ago but things stalled. I'm happy to
> pick this up again and have another go, but I wanted to ask first
> whether there was any objection to my converting tests to using tempfile
> functions which should avoid the problem?

I personally don't see any reason to block a move away from TESTFN since it
obviously has some inherent issues in parallel test running which means
flaky tests.
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