Performance of list vs. set equality operations

[Steven D'Aprano]

On Wed, 07 Apr 2010 10:55:10 -0700, Raymond Hettinger wrote:

> [Gustavo Nare]
>> In other words: The more different elements two collections have, the
>> faster it is to compare them as sets. And as a consequence, the more
>> equivalent elements two collections have, the faster it is to compare
>> them as lists.
>>
>> Is this correct?
> 
> If two collections are equal, then comparing them as a set is always
> slower than comparing them as a list.  Both have to call __eq__ for
> every element, but sets have to search for each element while lists can
> just iterate over consecutive pointers.
> 
> 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




-- 
Steven