Customizing sequence types

[Chris Rebert]

On Mon, Nov 17, 2008 at 10:05 AM, Mr. SpOOn <mr.spoon21 at gmail.com> wrote:
> It seems that I solved my main problem, but I still have some doubt.
>
> I'll make an example:
>
>>>> class foo:
> ...    def __init__(self, a):
> ...        self.a = a
> ...
>>>> f = foo(1)
>>>> f2 = foo(2)
>>>> f3 = foo(3)
>>>> f1 = foo(1)
>>>> s = set()
>>>> s.add(f)
>>>> s
> set([<__main__.foo instance at 0x8311fac>])
>>>> s.add(f2)
>>>> s.add(f3)
>>>> s.add(f1)
>>>> s
> set([<__main__.foo instance at 0x831934c>, <__main__.foo instance at
> 0x83191cc>, <__main__.foo instance at 0x8311fac>, <__main__.foo
> instance at 0x831932c>])
>
> I want that f and f1, that have both self.a set to 1, look the same to
> the set, so that it doesn't add f1. In this case the instances looks
> all different, so it adds them all.
>
> I tried rewriting __hash__ and __cmp__ in the foo class, so that
> __hash__ simply returns self.a and __cmp__ return self.a == other.a

__cmp__ does rich comparisons and is supposed to return 0 for
equality, -1 if the object is less than the other, and 1 if it's
greater than the other. So, using == as its definition is broken as it
returns just a boolean. You'd want cmp(self.a, other.a) if you were
defining __cmp__. However, == obviously works fine for __eq__, hence
why switching to __eq__ fixed your problem.

Also, __hash__  should probably delegate and be defined as
hash(self.a) rather than just self.a itself.

>
> I thought this would work, but I was wrong.
> I had to rewrite __eq__ with the same code of __cmp__
>
> Why it doesn't work with __cmp__ or __hash__ ?

Probably on account of one of the above errors I explained.

Cheers,
Chris
-- 
I wonder if there's a Mrs. FoRk...

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