Performance of list vs. set equality operations

[Terry Reedy]

On 4/10/2010 8:32 AM, Stefan Behnel wrote:
> Steven D'Aprano, 08.04.2010 03:41:
>> On Wed, 07 Apr 2010 10:55:10 -0700, Raymond Hettinger wrote:

>>> If the two collections have unequal sizes, then both ways immediately
>>> return unequal.
>>
>>
>> Perhaps I'm misinterpreting what you are saying, but I can't confirm that
>> behaviour, at least not for subclasses of list:
>>
>> >>> class MyList(list):
>> ... def __len__(self):
>> ... return self.n
>> ...
>> >>> L1 = MyList(range(10))
>> >>> L2 = MyList(range(10))
>> >>> L1.n = 9
>> >>> L2.n = 10
>> >>> L1 == L2
>> True
>> >>> len(L1) == len(L2)
>> False
>
> This code incorrectly assumes that overriding __len__ has an impact on
> the equality of two lists. If you want to influence the equality, you
> need to override __eq__. If you don't, the original implementation is
> free to do whatever it likes to determine if it is equal to another
> value or not. If it uses __len__ for that or not is only an
> implementation detail that can't be relied upon.

After reading the responses of both you and Raymond, I realized that a) 
there is a real difference between 'checking lengths' and 'calling 
__len__', which I (and apparently the example) had seen as the same and 
b) that the example shows that assuming that they are the same is a 
mistake. Thank you both for the clarification.

Terry Jan Reedy