[Python-Dev] Allowing to run certain regression tests in subprocesses

[Nick Coghlan]

On 4 Aug 2013 12:03, "Eli Bendersky" <eliben at gmail.com> wrote:
>
> On Sat, Aug 3, 2013 at 6:59 PM, Nick Coghlan <ncoghlan at gmail.com> wrote:
> >
> > On 4 Aug 2013 09:43, "Eli Bendersky" <eliben at gmail.com> wrote:
> >>
> >> Hi All,
> >>
> >> Today the issue of cross-test global env dependencies showed its ugly
> >> head again for me. I recall a previous discussion
> >> (http://mail.python.org/pipermail/python-dev/2013-January/123409.html)
> >> but there were many more over the years.
> >>
> >> The core problem is that some tests modify the global env
> >> (particularly importing modules) and this sometimes has adverse
> >> effects on other tests, because test.regrtest runs all tests in a
> >> single process. In the discussion linked above, the particular culprit
> >> test__all__ was judged as a candidate to be moved to a subprocess.
> >>
> >> I want to propose adding a capability to our test harness to run
> >> specific tests in subprocesses. Each test will have some simple way of
> >> asking to be run in a subprocess, and regrtest will concur (even when
> >> running -j1). test__all__ can go there, and it can help solve other
> >> problems.
> >>
> >> My particular case is trying to write a test for
> >> http://bugs.python.org/issue14988 - wherein I have to simulate a
> >> situation of non-existent pyexpat. It's not hard to write a test for
> >> it, but when run in tandem with other tests (where C extensions loaded
> >> pyexpat) it becomes seemingly impossible to set up. This should not be
> >> the case - there's nothing wrong with wanting to simulate this case,
> >> and there's nothing wrong in Python and the stdlib - it's purely an
> >> artifact of the way our regression suite works.
> >
> > I'm not actively opposed to the suggested idea, but is there a specific
> > reason "test.support.import_fresh_module" doesn't work for this test?
>
> I'm not an expert on this topic, but I believe there's a problem
> unloading code that was loaded by C extensions. import_fresh_module is
> thus powerless here (which also appears to be the case empirically).

Sure, it's just unusual to have a case where "importing is blocked by
adding None to sys.modules" differs from "not actually available", so I'd
like to understand the situation better.

Cheers,
Nick.
>
> Eli
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