relative speed of incremention syntaxes (or "i=i+1" VS "i+=1")

Richard D. Moores rdmoores at gmail.com
Mon Aug 22 11:55:11 CEST 2011


On Sun, Aug 21, 2011 at 18:12, Steven D'Aprano
<steve+comp.lang.python at pearwood.info> wrote:

> But really, we're talking about tiny differences in speed. Such trivial
> differences are at, or beyond, the limit of what can realistically be
> measured on a noisy PC running multiple processes (pretty much all PCs
> these days). Here are three runs of each on my computer:
>
>
> [steve at sylar python]$ python2.5 -m timeit -s 'n=0' 'n = n+1'
> 1000000 loops, best of 3: 0.508 usec per loop
> [steve at sylar python]$ python2.5 -m timeit -s 'n=0' 'n = n+1'
> 1000000 loops, best of 3: 0.587 usec per loop
> [steve at sylar python]$ python2.5 -m timeit -s 'n=0' 'n = n+1'
> 1000000 loops, best of 3: 0.251 usec per loop
>
>
> [steve at sylar python]$ python2.5 -m timeit -s 'n=0' 'n += 1'
> 1000000 loops, best of 3: 0.226 usec per loop
> [steve at sylar python]$ python2.5 -m timeit -s 'n=0' 'n += 1'
> 1000000 loops, best of 3: 0.494 usec per loop
> [steve at sylar python]$ python2.5 -m timeit -s 'n=0' 'n += 1'
> 1000000 loops, best of 3: 0.53 usec per loop
>
> Look at the variation between runs! About 130% variation between the fastest
> and slowest for each expression. And there's no reason to think that the
> fastest results shown is as fast as it can get. The time is dominated by
> noise, not the addition.
>
>
> For what it's worth, if I try it with a more recent Python:
>
> [steve at sylar python]$ python3.2 -m timeit -s 'n=0' 'n = n+1'
> 1000000 loops, best of 3: 0.221 usec per loop
> [steve at sylar python]$ python3.2 -m timeit -s 'n=0' 'n = n+1'
> 1000000 loops, best of 3: 0.202 usec per loop
> [steve at sylar python]$ python3.2 -m timeit -s 'n=0' 'n = n+1'
> 1000000 loops, best of 3: 0.244 usec per loop
>
> [steve at sylar python]$ python3.2 -m timeit -s 'n=0' 'n += 1'
> 1000000 loops, best of 3: 0.49 usec per loop
> [steve at sylar python]$ python3.2 -m timeit -s 'n=0' 'n += 1'
> 1000000 loops, best of 3: 0.176 usec per loop
> [steve at sylar python]$ python3.2 -m timeit -s 'n=0' 'n += 1'
> 1000000 loops, best of 3: 0.49 usec per loop
>
>
> I simply do not believe that we can justify making *any* claim about the
> relative speeds of n=n+1 and n+=1 other than "they are about the same". Any
> result you get, faster or slower, will depend more on chance than on any
> real or significant difference in the code.

I couldn't resist giving it a try. Using Python 3.2.1 on a 64-bit
Windows 7 machine with a 2.60 gigahertz AMD Athlon II X4 620
processor, I did 18 tests, alternating between n=n+1 and n+=1 (so 9
each).

The fastest for n+=1 was
C:\Windows\System32> python -m timeit  -r 3 -s "n=0" "n += 1"
10000000 loops, best of 3: 0.0879 usec per loop

The slowest for n+=1 was
C:\Windows\System32> python -m timeit  -r 3 -s "n=0" "n += 1"
10000000 loops, best of 3: 0.0902 usec per loop

The fastest for n = n + 1 was
C:\Windows\System32> python -m timeit  -r 3 -s "n=0" "n=n+1"
10000000 loops, best of 3: 0.0831 usec per loop

The slowest for n = n + 1 was
C:\Windows\System32> python -m timeit  -r 3 -s "n=0" "n=n+1"
10000000 loops, best of 3: 0.0842 usec per loop

Coincidence?

All the data are pasted at http://pastebin.com/jArPSe56

Dick Moores



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