[Python-Dev] Cleaning-up the new unittest API

Michael Foord fuzzyman at voidspace.org.uk
Sat Oct 30 05:56:04 CEST 2010

On 29/10/2010 23:29, Michael Foord wrote:
> [snip...]
>> Besides de-documenting those four redundant methods,
>> I propose that assertItemsEqual() be deprecated just like
>> its brother assertSameElements().  I haven't found anyone
>> who accurately guesses what those methods entail based
>> on their method names ("items" usually implies key/value
>> pairs elsewhere in the language; nor is it clear whether order is
>> important, whether the elements need to be hashable or
>> orderable or just define equality tests, nor is is clear whether
>> duplicates cause the test to fail).
>> Given the purpose of the unittest module, it's important that
>> the reader have a crystal clear understanding of what a test
>> is doing.  Their attention needs to be focused on the subject
>> of the test, not on questioning the semantics of the test method.
> assertItemsEqual compares two iterables and tests that they have the 
> same elements irrespective of order. A relatively straightforward 
> definition. Hopefully the docstring and documentation make this clear.
> If the members are all of the same type then indeed comparing two 
> sorted lists is only slightly more typing. If the members are of 
> different types checking that the members are the same is a much 
> harder problem in Python 3, and this method can be very useful.
Just to clarify. The following fails in Python 3:

     sorted([3, 1, 2, None])

If you want to compare that two iterables containing heterogeneous types 
have the same members then it is tricky to implement correctly and 
assertItemsEqual does it for you.

I agree that the name is not ideal and would be happy to change the name 
(deprecating the old name as it was released in 2.7). API churn is as 
bad as API bloat, but at least changing the name is something only done 

All the best,

> -1 for deprecating.
> All the best,
> Michael Foord
>> IMO, users are far better-off sticking with assertEqual() so they
>> can be specific about the nature of the test:
>>    # hashable elements; ignore dups
>>    assertEqual(set(a), set(b))
>>    # orderable elements; dups matter, order doesn't
>>    assertEqual(sorted(a), sorted(b))
>>    # eq tested elements, dups matter, order matters
>>    assertEqual(list(a), list(b))
>>    # hashable keys, eq tested values
>>    # ignore dups, ignore order
>>    assertEqual(dict(a), dict(b))
>> These take just a few more characters than assertSameElements()
>> and assertItemsEqual(), but they are far more clear about their meaning.
>> You won't have to second guess what semantics are hidden
>> behind the abstraction.
>> There are a couple other problems with the new API but it is probably
>> too late to do anything about it.
>> * elsewhere in Python we spell comparison names with abbreviations
>>    like eq, ne, lt, le, gt, ge.    In unittest, those are spelled in 
>> an awkward,
>>    not easily remembered manner:   assertLessEqual(a, b), etc.
>>    Fortunately, it's clear what the mean; however, it's not easy to 
>> guess
>>    their spelling.
>> * the names for assertRegexpMatches() and assertNotRegexpMatches
>>    are deeply misleading since they are implemented in terms of
>>    re.search(), not re.match().
>> Raymond
>> P.S.  I also looked ar assertDictContainsSubset(a,b).  It is a bit
>> over-specialized, but at least it is crystal clear what is does
>> and it beats the awkward alternative using dict views:
>>    assertLessEqual(a.items(), b.items())
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