[Python-Dev] Allowing to run certain regression tests in subprocesses (original) (raw)

Eli Bendersky eliben at gmail.com
Sun Aug 4 15:40:15 CEST 2013

On Sat, Aug 3, 2013 at 7:08 PM, Nick Coghlan <ncoghlan at gmail.com> wrote:

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.importfreshmodule" 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. importfreshmodule 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.

I must admit I'm confused by the behavior of import_fresh_module too.

Snippet #1 raises the expected ImportError:

sys.modules['pyexpat'] = None import _elementtree

However, snippet #2 succeeds importing:

ET = import_fresh_module('_elementtree', blocked=['pyexpat']) print(ET)

I believe this happens because import_fresh_module does an import of the 'name' it's given before even looking at the blocked list. Then, it assigns None to sys.modules for the blocked names and re-imports the module. So in essence, this is somewhat equivalent to snippet #3:

modname = '_elementtree' import(modname) del sys.modules[modname] for m in sys.modules: if modname == m or m.startswith(modname + '.'): del sys.modules[m] sys.modules['pyexpat'] = None ET = importlib.import_module(modname) print(ET)

Which also succeeds.

I fear I'm not familiar enough with the logic of importing to understand what's going on, but it has been my impression that this problem is occasionally encountered with import_fresh_module and C code that imports stuff (the import of pyexpat is done by C code in this case).

CC'ing Brett.

Eli

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