Would there be support for a more general cmp/__cmp__

Steve Holden steve at holdenweb.com
Thu Oct 20 13:32:35 CEST 2005


Antoon Pardon wrote:
> Op 2005-10-20, Steve Holden schreef <steve at holdenweb.com>:
> 
>>Antoon Pardon wrote:
>>
>>>I was wondering how people would feel if the cmp function and
>>>the __cmp__ method would be a bit more generalised.
>>>
>>>The problem now is that the cmp protocol has no way to
>>>indicate two objects are incomparable, they are not
>>>equal but neither is one less or greater than the other.
>>>
>>>So I thought that either cmp could return None in this
>>>case or throw a specific exception. People writing a
>>>__cmp__ method could do the same.
>>>
>>
>>The current behaviour is, of course, by design: """The operators <, >, 
>>==, >=, <=, and != compare the values of two objects. The objects need 
>>not have the same type. If both are numbers, they are converted to a 
>>common type. Otherwise, objects of different types always compare 
>>unequal, and are ordered consistently but arbitrarily."""
>>
>>Personally I'm still not convinced that your requirement reflects a 
>>substantial use case (but then I'm getting used to that ;-). Just 
>>because an ordering is partial that doesn't mean that two instances of a 
>>class shouldn't be compared.
> 
> 
> I'm not suggesting that they shouldn't be compared.
> 
Sorry, I thought that was the meaning of "The problem now is that the 
cmp protocol has no way to indicate two objects are incomparable".
> 
>>What would you have Python do when the programmer tries to perform an 
>>invalid comparison (i.e. what are the exact semantics imposed when 
>>__cmp__() returns None/raises an exception)?
> 
> 
> My appologies, I should have been more complete.
> 
> What I want is a way to say that all of the following are False:
> 
>   a < b, a <= b, a == b, a > b, a >= b
> 
> and that only the following is True:
> 
>   a != b
> 
> 
> So if a coder writes one of the comparisons and as a result python
> calls the __cmp__ method on one of the terms which would return
> either None or raise an exceptions I would like it to reflect
> the above behaviour.
> 
Ergo: use rich comparisons.

regards
  Steve
-- 
Steve Holden       +44 150 684 7255  +1 800 494 3119
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