while True or while 1

[Terry Reedy]

On 12/16/2010 7:23 AM, Steve Holden wrote:
> On 12/16/2010 5:44 AM, BartC wrote:

>> One these is 30% faster than the other. That's an appreciable
>> difference, which you can't really just dismiss.
>>
>> And you can't tell what the overall effect on a program will be: perhaps
>> the loop will be in a library function , which might be called billions
>> of times.
>
> It might. But if the code it is calling does *anything* at all
> significant I can promise you that spending an extra 30% purely on the
> looping construct will still make a negligible difference to a program's
> execution time, and there are almost certainly going to be many other
> aspects of performance that will yield greater benefits from tuning.
>
> I realise that there are going to be some people who just flatly say
> "since 'while 1:' is quicker I am going to use it every time", and that
> their programs will still work. And I still maintain that (for English
> speakers) "while True:" is to be preferred.
>
> sholden at lifeboy ~
> $ python -m timeit -- "i = 1" "while True:" "    i += 1" "    if i ==
> 1000000: break"
> 10 loops, best of 3: 157 msec per loop
>
> sholden at lifeboy ~
> $ python -m timeit -- "i = 1" "while True:" "    i += 1" "    if i ==
> 1000000: break" "    x = i+1"
> 10 loops, best of 3: 238 msec per loop
>
> sholden at lifeboy ~
> $ python -m timeit -- "i = 1" "while 1:" "    i += 1" "    if i ==
> 1000000: break"
> 10 loops, best of 3: 116 msec per loop
>
> sholden at lifeboy ~
> $ python -m timeit -- "i = 1" "while 1:" "    i += 1" "    if i ==
> 1000000: break" "    x = i+1"
> 10 loops, best of 3: 195 msec per loop

This are all Python2 timings.

> If binding a simple arithmetic expression adds more to the loop than the
> difference between "while 1:" and "while True:" you are wasting your
> time thinking about the savings under all but the most rigorous
> circumstances.

*Especially* when the 'problem' has been fixed in Python 3.

-- 
Terry Jan Reedy