[Tutor] Making a function run every second.

[Dave Angel]

On 11/29/2011 11:09 AM, Dave Angel wrote:
> On 11/29/2011 10:52 AM, Mic wrote:
>>
>> <SNIP>
>> Okay, I undestand. Hmm, what is a drift?
>>
I just noticed I hadn't answered that one.  It doesn't matter any more 
since you're running tkinter.

But for completeness:

If you had a non-event driven program (eg. a console app), and you had a 
loop like the following:


SLEEPTIME = 1
while True:
     sleep(SLEEPTIME)
     myfunc()

myfunc() would  be called about once a second.  However, in a heavily 
loaded system, it might be 1.2 seconds one time, and 1.1 the next.  So 
it might be called only 50 times in that minute, rather than 60.  I 
believe that in some OS's, it might be called 75 times in a minute.  
Anyway, if drift is to be avoided, you need to keep track of the time 
that the next call should be made, compare it to "now", and calculate 
what to pass as a parameter to sleep().

In untested approx. code,

import time
import itertools
starttime = float(now())
SLEEPTIME = 1   (for one second)

for intervalnum in itertools.count():
       delayneeded = starttime + SLEEPTIME*intervalnum - now()
       time.sleep(delayneeded)
       myfunc()

if this drifts, it tries to compensate on the next iteration.  Note that 
there are cleverer implementations of this fundamental scheme, depending 
on other requirements or goals.

-- 

DaveA