List Performance

[Maric Michaud]

Le Monday 30 June 2008 22:21:35 Terry Reedy, vous avez écrit :
> > Well, as I posted few days ago, one could envisage, as a pure python
> > optimization for dealing with long list, to replace an algorithm with a
> > lot of append by something like this :
> >
> > mark = object()
> >
> > datas = [ mark ] * expected_size
>
> datas = [None] * expected_size
> has been a standard idiom since before object() existed ;-)
> and works fine *unless* one wants to add None explicitly
> and have that be different from 'unused'.


Yes, in fact I used a marker because it I thought of it primarily as outbound 
for the list (like \0 for strings in C), but it doesnt' matter what is the 
object you put in the list, if you know at every moment its size.

A subclass of list will indeed have to override most of the methods of its 
parent (not just some as I assumed before), using extend for reallocation 
with some sort of iterator with size, as it work in my previous example with 
xrange, something like that :

>>>[31]: class iter_with_len(object) :
    def __init__(self, size, obj=None) :
        self.size = size
        self.obj = obj
    def __len__(self) : return self.size
    def __iter__(self) : return itertools.repeat(self.obj, len(self))
   ....:
   ....:




-- 
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Maric Michaud