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

[Michael Foord]

On 02/11/2010 17:17, exarkun at twistedmatrix.com wrote:
> On 04:29 pm, fuzzyman at voidspace.org.uk wrote:
>> On 02/11/2010 16:23, Terry Reedy wrote:
>>> On 11/2/2010 10:05 AM, C. Titus Brown wrote:
>>>> ...but, as someone who has to figure out how to teach stuff to CSE 
>>>> undergrads
>>>> (and biology grads) I hate the statement "...any programmer should
>>>> expect this..."
>>>
>>> And indeed I (intentionally) did not say that. People who are 
>>> ignorant and inexperienced about something should avoid making 
>>> expectations in any direction until they have read the doc and 
>>> experimented a bit.
>> Expectations come from consistent behaviour. sorted behaves 
>> consistently for *most* of the built-in types and will also work for 
>> custom types that provide a 'standard' (total ordering) 
>> implementation of __lt__.
>>
>> It is very easy to *not realise* that a consequence of sets (and 
>> frozensets) providing partial ordering through operator overloading 
>> is that sorting is undefined for them.
>
> Perhaps. The documentation for sets says this, though:
>
> Since sets only define partial ordering (subset relationships), the 
> output of the list.sort() method is undefined for lists of sets.

Right, I did quote that exact text earlier in the thread. False 
expectations come when there are exceptions to otherwise-consistent 
behaviour.

>> Particularly as it still works for other mutable collections. Worth 
>> being aware that custom implementations of standard operators will 
>> break expectations of users who aren't intimately aware of the 
>> problem domains that the specific type may be created for.
>
> I can't help thinking that most of this confusion is caused by using < 
> for determining subsets. If < were not defined for sets and people had 
> to use "set.issubset" (which exists already), then sorting a list with 
> sets would raise an exception, a much more understandable failure mode 
> than getting back a list in arbitrary order.
>
I agree. This is a cost of overloading operators with domain specific 
meanings.

All the best,

Michael Foord


> Jean-Paul
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