Chris Angelico wrote:
> On Fri, Dec 9, 2011 at 4:32 AM, Peter Otten <__pete...@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>}
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