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[Python-ideas] Implement comparison operators for range objects

Raymond Hettinger raymond.hettinger at gmail.com
Thu Oct 13 06:27:21 CEST 2011 
On Oct 12, 2011, at 12:36 PM, Sven Marnach wrote:

> Steven D'Aprano schrieb am Do, 13. Okt 2011, um 04:33:49 +1100:
>>> When implementing '==' and '!=' for range objects, it would be natural
>>> to implement the other comparison operators, too (lexicographically,
>>> as for all other sequence types).
>> 
>> I don't agree. Equality makes sense for ranges: two ranges are equal
>> if they have the same start, stop and step values. 
> 
> No, two ranges should be equal if they represent the same sequence,
> i.e. if they compare equal when converted to a list:
> 
>    range(0) == range(4, 4, 4)
>    range(5, 10, 3) == range(5, 11, 3)
>    range(3, 6, 3) == range(3, 4)

Given that there are two reasonably valid interpretations of equality (i.e. produces-equivalent-sequences or the stronger condition, has-an-equal-start/stop/step-tuple), we should acknowledge the ambiguity and refuse the temptation to guess.   I vote for not defining equality for range objects -- it's not really an important service anyway (you really can live without it).

Instead, let people explicitly compare the raw components, or if desired, compare components normalized by a slice:

   >>> range(2, 9, 2)[:]
   range(2, 10, 2)


Raymond
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