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From: Peter Otten <__peter__@web.de>
Subject: Re: adding elements to set
Date: Thu, 08 Dec 2011 19:43:01 +0100
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Xref: x330-a1.tempe.blueboxinc.net comp.lang.python:16860

Chris Angelico wrote:

> On Fri, Dec 9, 2011 at 4:32 AM, Peter Otten <__peter__@web.de> wrote:
>> The only thing that has changed (in 2.7) is the algorithm to calculate
>> the hash value. The bits are rotated to turn the four least significant
>> bits into the most signicant ones. According to a comment in
>> Objects/objects.c the change leads to fewer hash collisions.
> 
> Interesting, but what I saw was this:
> 
>>>> class C(object):
> 
>    def __init__(self, x):
>        self.x = x
> 
>    def __eq__(self, other):
>        return self.x == other.x
> 
>>>> s=set()
>>>> c1=C(1)
>>>> s.add(c1)
> Traceback (most recent call last):
>   File "<pyshell#163>", line 1, in <module>
>     s.add(c1)
> TypeError: unhashable type: 'C'
> 
> (This is in IDLE from Python 3.2 on Windows.)
> 
> However, s.add(object()) works fine. So subclasses don't get that.
> Odd. Makes sense though - you can't get this unexpected behaviour as
> easily.

It seems to be even subtler: you can subclass if you don't implement 
__eq__():

>>> class C(object): pass
...
>>> {C()}
{<__main__.C object at 0x17defd0>}