[pypy-dev] Copy of list

[Armin Rigo]

Hi Tuom,

On Mon, Sep 28, 2015 at 3:57 PM, Tuom Larsen <tuom.larsen at gmail.com> wrote:
>     from timeit import timeit
>     print timeit('b = a[:]', 'a = list(range(100))')    # 0.0732719898224
>     print timeit('b = list(a)', 'a = list(range(100))') # 0.00285792350769

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.


A bientôt,

Armin.