[Python-Dev] Using more specific methods in Python unit tests

Jeff Allen ja.py at farowl.co.uk
Sun Feb 16 11:23:39 CET 2014

On 16/02/2014 00:22, Nick Coghlan wrote:
> On 16 February 2014 09:20, Ned Deily <nad at acm.org> wrote:
>> In article
>> <1392492250.26338.83831085.39A5ED08 at webmail.messagingengine.com>,
>>   Benjamin Peterson <benjamin at python.org> wrote:
>>> On Sat, Feb 15, 2014, at 10:12 AM, Serhiy Storchaka wrote:
>>>> Although Raymond approved a patch for test_bigmem [2], his expressed the
>>>> insistent recommendation not to do this. So I stop committing new
>>>> reviewed patches. Terry recommended to discuss this in Python-Dev. What
>>>> are your thoughts?
>>> I tend to agree with Raymond. I think such changes are very welcome when
>>> the module or tests are otherwise being changed, but on their on
>>> constitute unnecessary churn.
> Right, there are a few key problems with large scale style changes to
> the test suite:
> 1. The worst case scenario is where we subtly change a test so that it
> is no longer testing what it is supposed to be testing, allowing the
> future introduction of an undetected regression. This isn't
> particularly *likely*, but a serious problem if it happens.
> 2. If there are pending patches for that module that include new
> tests, then the style change may cause the patches to no longer apply
> cleanly, require rework of bug fix and new feature patches to
> accommodate the style change.
> 3. Merges between branches may become more complicated (for reasons
> similar to 2), unless the style change is also applied to the
> maintenance branches (which is problematic due to 1).
I spend a *lot* of time working with the Python test suite on behalf of 
Jython, so I appreciate the care CPython puts into its testing. To a 
large extent, tests written for CPython drive Jython development: I 
suspect I work with a lot more failing tests than anyone here. Where we 
have a custom test, I often update them from in the latest CPython tests.

Often a test failure is not caused by code I just wrote, but by adding a 
CPython test or removing a "skip if Jython", and not having written 
anything yet. While the irrefutable "False is not true" always raises a 
smile, I'd welcome something more informative. It's a more than a 
"style" issue.

What Nick says above is also not false, as general guidance, but taken 
as an iron rule seem to argue against concurrent development at all. 
Don't we manage this change pretty well already? I see little risk of 
problems 1-3 in the actual proposal, as the changes themselves are 99% 
of the "drop-in replacement" type:

-        self.assertTrue(isinstance(x, int))
+        self.assertIsInstance(x, int)

I found few places, on a quick scan, that risked changing the meaning: 
they introduce an if-statement, or refactor the expression -- I don't 
mean they're actually wrong. The point about breaking Serhiy's patch 
into independent parts will help manage with merging and this risk.

The tests are not library code, but their other use is as an example of 
good practice in unit testing. I pretty much get my idea of Python's 
test facilities from this work. It was a while before I realised more 
expressive methods were available.


Jeff Allen

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