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
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