list.clear() missing?!?

[Felipe Almeida Lessa]

Em Qua, 2006-04-12 às 11:36 +1000, Steven D'Aprano escreveu:
> On Tue, 11 Apr 2006 19:15:18 +0200, Martin v. Löwis wrote:
> 
> > Felipe Almeida Lessa wrote:
> >> I love benchmarks, so as I was testing the options, I saw something very
> >> strange:
> >> 
> >> $ python2.4 -mtimeit 'x = range(100000); '
> >> 100 loops, best of 3: 6.7 msec per loop
> >> $ python2.4 -mtimeit 'x = range(100000); del x[:]'
> >> 100 loops, best of 3: 6.35 msec per loop
> >> $ python2.4 -mtimeit 'x = range(100000); x[:] = []'
> >> 100 loops, best of 3: 6.36 msec per loop
> >> $ python2.4 -mtimeit 'x = range(100000); del x'
> >> 100 loops, best of 3: 6.46 msec per loop
> >> 
> >> Why the first benchmark is the slowest? I don't get it... could someone
> >> test this, too?
> > 
> > In the first benchmark, you need space for two lists: the old one and
> > the new one; 
> 
> Er, what new list? I see only one list, x = range(100000), which is merely
> created then nothing done to it. Have I missed something?

He's talking about the garbage collector.

Talking about the GC, do you want to see something *really* odd?

$ python2.4 -mtimeit -s 'from gc import collect' 'collect(); x =
range(100000); '
100 loops, best of 3: 13 msec per loop

$ python2.4 -mtimeit -s 'from gc import collect' 'collect(); x =
range(100000); del x[:]'
100 loops, best of 3: 8.19 msec per loop

$ python2.4 -mtimeit -s 'from gc import collect' 'collect(); x =
range(100000); x[:] = []'
100 loops, best of 3: 8.16 msec per loop

$ python2.4 -mtimeit -s 'from gc import collect' 'collect(); x =
range(100000); del x'
100 loops, best of 3: 8.3 msec per loop


But in this case I got the answer (I think):
- When you explicitly delete the objects, the GC already know that it
can be collected, so it just throw the objects away.
- When we let the "x" variable continue to survive, the GC has to look
at all the 100001 objects to see if they can be collected -- just to see
that it can't.

Also, IIRC "del x" is slower than "x = []" because removing a name from
the namespace is more expensive than just assigning something else to
it. Right?

> I understood Felipe to be asking, why does it take longer to just create a
> list, than it takes to create a list AND then do something to it? 

I see dead people... ;-)

-- 
Felipe.