Peter Otten <__pete...@web.de>:
> Marko Rauhamaa wrote:
>> I see no point in CPython's rotation magic.
>
> Let's see:
>
> $ cat hashperf.py
> class A(object):
> __slots__ = ["_hash"]
>
> def __hash__(self):
> return self._hash
>
> def no_magic():
> a = A()
> a._hash = id(a)
> return a
>
> def magic():
> a = A()
> a._hash = id(a) >> 4
> return a
>
> $ python3 -m timeit -s 'from hashperf import magic, no_magic; s =
> {no_magic() for _ in range(10**5)}' 'for x in s: x in s'
> 10 loops, best of 3: 70.7 msec per loop
>
> $ python3 -m timeit -s 'from hashperf import magic, no_magic; s = {magic()
> for _ in range(10**5)}' 'for x in s: x in s'
> 10 loops, best of 3: 52.8 msec per loop
>
> "magic" wins this makeshift test. Other than that you're right ;)
That's interesting, but suggests there's something weird (~ suboptimal)
going on with CPython's scrambling algorithm. Also, your performance
test might yield completely different results on other Python
implementations.
Apart from uniqueness, there's no particular reason to prefer one
__hash__() value over another as long as the interesting bits are inside
the CPU's simple integer range.
Marko
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