How to let a loop run for a while before checking for break condition?

[Diez B. Roggisch]

Fredrik Lundh schrieb:
> Diez B. Roggisch wrote:
> 
>> A while loop has a condition. period. The only thing to change that is 
>> to introduce a uncoditioned loop, and use self-modifying code to make 
>> it a while-loop after that timer interrupt of yours.
> 
> or use a timer interrupt to interrupt the loop:
> 
> import signal, time
> 
> def func1(timeout):
> 
>     def callback(signum, frame):
>         raise EOFError # could use a custom exception instead
>     signal.signal(signal.SIGALRM, callback)
>     signal.alarm(timeout)
> 
>     count = 0
>     try:
>         while 1:
>             count += 1
>     except EOFError:
>         for i in range(10):
>             count += 1
>     print count
> 
> for an utterly trivial task like the one in that example, the alarm 
> version runs about five times faster than a polling version, on my test 
> machine (ymmv):

No doubt that changing the flag asynchronously is a gain by delegating 
the timing code to the OS. Yet the while loop still has a condition - so 
you could as well set a flag in the signal handler an do it like this:

def func3(timeout):
     global flag
     flag = True
     def callback(signum, frame):
         global flag
         flag = False
     signal.signal(signal.SIGALRM, callback)
     signal.alarm(timeout)

     count = 0

     while flag or True:
         count += 1


     for i in range(10):
         count += 1
     print count

This is on my machine about 1.5 times slower than func1, but much more 
readable especially wrt the OPs request of a condition being evaluated 
after a certain timeout, as you don't repeat any code.

And apart from that, the overall gain of performance diminishes the very 
moment anything non-trivial occurs in the loops body anyway.

Diez