Basic question about speed/coding style/memory
janpeterr at freenet.de
Sat Jul 21 11:32:53 CEST 2012
On 21.07.2012 12:06, Steven D'Aprano wrote:
> But in general, you're worrying too much about trivia. One way or the
> other, any speed difference will be trivial. Write whatever style reads
> and writes most naturally, and only worry about what's faster where it
> actually counts.
> Notice that I try to make each function do the same amount of work, so
> that we're seeing only the difference between "else" vs "no else".
> Now let's test the speed difference with Python 2.7. Because this is
> timing small code snippets, we should use the timeit module to time the
> from timeit import Timer
> setup = "from __main__ import with_else, without_else"
> t1 = Timer("for i in (0, 1): result = with_else(i)", setup)
> t2 = Timer("for i in (0, 1): result = without_else(i)", setup)
> Each snippet calls the function twice, once to take the if branch, then
> to take the else branch.
> Now we time how long it takes to run each code snippet 1000000 times. We
> do that six times each, and print the best (lowest) speed:
> py> min(t1.repeat(repeat=6))
> py> min(t2.repeat(repeat=6))
> So there is approximately 0.03 second difference per TWO MILLION
> if...else blocks, or about 15 nanoseconds each. This is highly unlikely
> to be the bottleneck in your code. Assuming the difference is real, and
> not just measurement error, the difference is insignificant.
> So, don't worry about which is faster. Write whichever is more natural,
> easier to read and write.
very nice example and thank you very much for also for the Timeit test!
Actually it confirms my assumption in some way:
So if there is some overhead in some fashion in case we don't offer the
else, assuming the interpreter has to exit the evaluation of the
"if"-statement clause and return to a "normal parsing code"-state
outside the if statement itself.
Without having looked at Andrew's bytecode excecution hint, using the
dis module, to see how the interpreter handles the task on lower level.
But fare enough for me :)
But I agree, the return in my example is misleading and it would be
illegal outside of a function call. I just added it to make clear that
the fellow code below the return should not be executed in comparison to
the 2nd example.
Thank you very much
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