How do I ensure that a Python while-loop takes a particular amount of time to run?

Question

I\'m reading serial data with a while loop. However, I have no control over the sample rate.

The code itself seems to take 0.2s to run, so I know I won\'t be able to

Answer 1

At the beginning of the loop check if the appropriate amount of time has passed. If it has not, sleep.

# Set up initial conditions for sample_time outside the loop
sample_period = ???
next_min_time = 0
while True:
    sample_time = time.time() - zero
    if sample_time < next_min_time:
        time.sleep(next_min_time - sample_time)
        continue
    # read and print a line
    sample_value = ser.readline()
    sample_line = str(sample_time)+','+str(sample_value)
    outfile.write(sample_line)
    print 'time: {}, value: {}'.format(sample_time, sample_value)
    next_min_time = sample_time + sample_period

Answer 2

Just measure the time running your code takes every iteration of the loop, and sleep accordingly:

import time

while True:
    now = time.time()            # get the time
    do_something()               # do your stuff
    elapsed = time.time() - now  # how long was it running?
    time.sleep(1.-elapsed)       # sleep accordingly so the full iteration takes 1 second

Of course not 100% perfect (maybe off one millisecond or another from time to time), but I guess it's good enough.

---

Another nice approach is using twisted's LoopingCall:

from twisted.internet import task
from twisted.internet import reactor

def do_something():
    pass # do your work here

task.LoopingCall(do_something).start(1.0)
reactor.run()

Answer 3

An rather elegant method is you're working on UNIX : use the signal library

The code :

import signal


def _handle_timeout():
    print "timeout hit" # Do nothing here

def second(count):
    signal.signal(signal.SIGALRM, _handle_timeout)
    signal.alarm(1)
    try:
        count += 1 # put your function here
        signal.pause()

    finally:
        signal.alarm(0)
        return count


if __name__ == '__main__':

    count = 0
    count = second(count)
    count = second(count)
    count = second(count)
    count = second(count)
    count = second(count)

    print count

And the timing :

 georgesl@cleese:~/Bureau$ time python timer.py
 5

 real   0m5.081s
 user   0m0.068s
 sys    0m0.004s

Two caveats though : it only works on *nix, and it is not multithread-safe.