[Python-ideas] Length hinting and preallocation for container types

Philip Jenvey pjenvey at underboss.org
Wed Mar 6 02:06:25 CET 2013

On Tue, Mar 5, 2013 at 2:03 PM, Alex Stewart <foogod at gmail.com> wrote:

> On Tue, Mar 5, 2013 at 12:42 PM, Andrew Barnert <abarnert at yahoo.com>wrote:
>> So, that's a 1.8% speedup. While doing things right gives a 20% speedup:
>> $ python3.3 -m timeit -n1000 "l=[]" "app = l.append" "for i in
>> range(10000): app(i)"
>> 1000 loops, best of 3: 557 usec per loop
>> $ python3.3 -m timeit -n1000 "l = [i for i in range(10000)]"
>> 1000 loops, best of 3: 447 usec per loop
> Yeah, I think it's pretty likely that any case where the proposed change
> would be applicable is probably going to be dominated by other factors more
> than by allocation overhead anyway..
> The above does beg the question, though:  Could we perhaps apply some of
> this thinking to list comprehensions, and speed them up even more by
> automatically taking hints from the inputs and preallocating the output
> list? (or do we already?)
> Not sure how much work that would be, or whether it would be worth it..
> just a random thought..

This was proposed here:


Philip Jenvey
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