[Python-Dev] test___all__ polluting sys.modules? (original) (raw)

Eli Bendersky eliben at gmail.com
Sun Dec 30 14:33:56 CET 2012

On Sat, Dec 29, 2012 at 5:34 PM, Nick Coghlan <ncoghlan at gmail.com> wrote:

On Sun, Dec 30, 2012 at 10:46 AM, Benjamin Peterson <benjamin at python.org> wrote: > 2012/12/29 Eli Bendersky <eliben at gmail.com>: >> Hi, >> >> This came up while investigating some test-order-dependency failures in >> issue 16076. >> >> test_all goes over modules that have _all_ in them and does_ 'from >> import *' on them. This leaves a lot of modules in sys.modules, >> which may interfere with some tests that do fancy things with sys,modules. >> In particular, the ElementTree tests have trouble with it because they >> carefully set up the imports to get the C or the Python version of etree >> (see issues 15083 and 15075). >> >> Would it make sense to save the sys.modules state and restore it in >> test_all so that sys.modules isn't affected by this test?_ > > Sounds reasonable to me.

I've tried this as an inherent property of regrtest before (to resolve some problems with testpydoc), and it didn't work - we have too many modules with non-local side effects (IIRC, mostly related to the copy and pickle registries). Given that it checks the whole standard library, test_all is_ likely to run into the same problem. Yes, I'm running into all kinds of weird problems when saving/restoring sys.modules around test___all__. This is not the first time I get to fight this test-run-dependency problem and it's very frustrating.

This may be a naive question, but why don't we run the tests in separate interpreters? For example with -j we do (which creates all kinds of strange intermittent problems depending on which tests got bundled into the same process). Is this a matter of performance? Because that would help get rid of these dependencies between tests, which would probably save core devs some work and headache.

After all, since a lot of the interpreter state is global (for example sys.modules), does it not make sense to run each test in a clean environment? Many tests do fancy things with the global environment which makes them difficult to keep clean and separate.

Hence test.support.importfreshmodule - it can ensure you get the module you want, regardless of the preexisting contents of sys.modules. ( http://docs.python.org/dev/library/test#test.support.importfreshmodule)

Yes, this is the solution currently used in test_xml_etree. However, once pickling tests are added things stop working. Pickle uses import to import the module a class belongs to, bypassing all such trickery. So if test___all__ got _elementtree into sys.modules, pickle's import finds it even if all the tests in test_xml_etree manage to ignore it for the Python version because they use import_fresh_module.

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