The Cost of Dynamism (was Re: Pyhon 2.x or 3.x, which is faster?)

Thu Mar 24 10:16:06 EDT 2016

On 24/03/2016 14:08, Jon Ribbens wrote:
> On 2016年03月24日, BartC <bc at freeuk.com> wrote:
>> On 24/03/2016 13:50, Steven D'Aprano wrote:
>>> Likewise clearing a list:
>>>>>> for i in range(len(mylist)-1, -1, 0):
>>> del mylist[i]
>>>> That's wouldn't be I'd call clearing a list, more like destroying it
>> completely!
>>>> How would you actually clear a list by traversing it (ie. not just
>> building a new one)?
>>>> This doesn't work:
>>>> for x in L:
>> x=0
>>>> as each x only refers to the value in each element of L, not the element
>> itself (like the pass-by-reference problem).
>>>> I'd presumably have to do:
>>>> for i in range(len(L)):
>> L[i]=0
>> That's kind've a weird thing to want to do;

The thing I'm trying to demonstrate is changing an element of a list 
that you are traversing in a loop. Not necessarily set all elements to 
the same value.
 if you thought you needed
> to do that then most likely what you should actually be doing is
> re-writing your code so you no longer need to. However, you could do:
>> L[:] = [0] * len(L)

OK, but that's just building a new list as I've already mentioned.
Or is the Pythonic way, when you want to change some elements of a list 
to just build a new one that incorporates those changes?
-- 
Bartc