Python 3 __cmp__ semantic change?

[Steven D'Aprano]

On Sat, 22 Nov 2008 08:27:59 +0000, Arnaud Delobelle wrote:

>>> Granted it's not as efficient as a __cmp__ function.
>>
>> What makes you say that? What do you mean by "efficient"? Are you
>> talking about memory footprint, runtime speed, disk-space, programmer
>> efficiency, algorithmic complexity, or something else?
> 
> What I'm talking about is very simple - and explained below, with the
> help of your __cmp__ method.
>>
>> As I see it, a __cmp__ method would be written something like this:
>>
>>     def __cmp__(self, other):
>>         return cmp(self.num*other.den, self.den*other.num)
> 
> I'm talking about runtime speed (*not* asymptotic complexity).  My code
> makes Fraction.__gt__ about twice as slow as Fraction.__lt__ or
> Fraction.__eq__ even though with __cmp__ they would all be equally fast.


Sounds like a premature micro-optimization to me. On my machine, running 
Python 2.5, the speed difference is nothing like twice as slow.


>>> class UseCmp(object):
...     def __init__(self, num, den=1):
...             self.num = num
...             self.den = den
...     def __cmp__(self, other):
...             return cmp(self.num*other.den, self.den*other.num)
...
>>> class UseRichCmp(UseCmp):
...     __lt__ = lambda self, other: self.__cmp__(other) < 0
...
>>> from timeit import Timer
>>> t1 = Timer('x < y',
... 'from __main__ import UseCmp; x=UseCmp(3, 5); y=UseCmp(1, 2)')
>>> t2 = Timer('x < y',
... 'from __main__ import UseRichCmp;'
... 'x=UseRichCmp(3, 5); y=UseRichCmp(1, 2)')
>>>
>>> t1.repeat()
[3.3418200016021729, 2.4046459197998047, 2.2295808792114258]
>>> t2.repeat()
[3.8954730033874512, 3.0240590572357178, 3.5528950691223145]

There's a slight speed difference, around 35% slower. But the random 
variation in speed is almost 50%, so I would conclude from this trial 
that there is no *significant* speed difference between the methods.


-- 
Steven