Hello Armin,
Thanks for the answer!
On Mon, Sep 28, 2015 at 8:17 PM, Armin Rigo <ar...@tunes.org> wrote:
>
> In addition to the other comments, this is not testing what you think
> it is: 'range(100)' returns a list optimized for being a range, and
> 'list(range(100))' does it too. You can see it like this:
>
>>>>> import __pypy__
>>>>> __pypy__.strategy([])
> 'EmptyListStrategy'
>>>>> __pypy__.strategy(range(100))
> 'SimpleRangeListStrategy'
>>>>> __pypy__.strategy(list(range(100)))
> 'SimpleRangeListStrategy'
>
> The SimpleRangeListStrategy means that the list is implemented as just
> a number, here 100.
>
>>>>> __pypy__.strategy(range(100)[:])
> 'IntegerListStrategy'
>
> So we discover here that slicing a range-list is not as optimized as
> it could be. It expands it to an array containing all the integers.
> That's the reason for why the slicing version seems slower in your
> tests. But optimizing that would of course be pointless in almost any
> real program.
However, I still get "slice-way" twice as slow as "constructor-way"
when I use normal lists:
from timeit import timeit
print timeit('b = a[:]', 'a = [1,2,3,4,5,6,7,8,9,0]') # 0.0329349040985
print timeit('b = list(a)', 'a = [1,2,3,4,5,6,7,8,9,0]') # 0.0171940326691
or:
pypy -m timeit -s 'a = [1,2,3,4,5,6,7,8,9,0]' 'b = a[:]' # 0.0306
usec per loop
pypy -m timeit -s 'a = [1,2,3,4,5,6,7,8,9,0]' 'b = list(a)' #
0.0149 usec per loop
Please, let me rephrase my question: currently I use `[:]` because it
is faster in CPython (0.131 usec vs 0.269 usec per loop). I almost
don't mind changing it to `list()` because of PyPy but I was wondering
what do PyPy developers recommend. I don't understand why is `[:]`
twice as slow as `list()` as it seems it should do the same thing
(create a list and copy the content). So my question is, is there a
reason why `list()` is fundamentally faster, and then I would switch
to it, or maybe/perhaps/somewhere-in-the-future PyPy would make it as
fast as `[:]`, as it does the same amount of work, and then I could
leave it as it is (`[:]`)?
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