Planning a Python Course for Beginners

[Marko Rauhamaa]

Peter Otten <__peter__ at 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