[Python-Dev] Problems with regrtest and with logging (original) (raw)

Nick Coghlan ncoghlan at gmail.com
Mon May 9 17:00:38 CEST 2011

On Sat, May 7, 2011 at 3:51 AM, Éric Araujo <merwok at netwok.org> wrote:

regrtest helpfully reports when a test leaves the environment unclean (sys.path, os.environ, logging.handlerList), but I think the implementation is buggy: it compares object identity and then value.  Why is comparing identity useful?  I’d just use ==.  It makes writing cleanup code easier (just use addCleanup(setattr, obj, 'attr', copy(obj.attr))).

Because changing the identity of any of those global state attributes that regrtest monitors is itself suggestive of a bug. When it comes to containers, identity matters at least as much as value does (and sometimes more so - e.g. sys.modules). Replacing those global containers with new ones isn't guaranteed to work, as they may be cached in various places rather than always retrieved fresh from the relevant module namespace. Modifying them in place, on the other hand, does the right thing even in the presence of cached references.

A comment to that effect may be a useful addition to regrtest, as I expect others may have similar questions about those identity checks in the future. (It may even be a useful addition to the documentation, but I have no idea where it could be sensibly included).

Also, don't be surprised if wholesale cleanup like that isn't completely reliable - it's far, far better if the test case understands the changes it is making (even indirectly) and explicitly reverses them. Save-and-restore should be a last resort technique (although context managers that are designed for more general use, such as warnings.catch_warnings(), use save-and-restore by necessity, since they have no control over the body of the relevant with statements).

Cheers, Nick.

-- Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia

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