Executing periodic actions in Python

Question

I am working on Windows  I want to execute a function foo   every 10 seconds   How do I do this

User · Accepted Answer

At the end of foo    create a Timer which calls foo   itself after 10 seconds  Because  Timer create a new thread to call foo     You can do other stuff without being blocked   import time  threading def foo        print time ctime        threading Timer 10  foo  start    foo     output   Thu Dec 22 14 46 08 2011  Thu Dec 22 14 46 18 2011  Thu Dec 22 14 46 28 2011  Thu Dec 22 14 46 38 2011

User · Answer

Simply sleeping for 10 seconds or using threading Timer 10 foo  will result in start time drift   You may not care about this  or it may be a significant source of problems depending on your exact situation   There can be two causes for this - inaccuracies in the wake up time of your thread or execution time for your function    You can see some results at the end of this post  but first an example of how to fix it  You need to track when your function should next be called as opposed to when it actually got called and account for the difference    Here s a version that drifts slightly   import datetime  threading  def foo        print datetime datetime now       threading Timer 1  foo  start    foo     Its output looks like this   2013-08-12 13 05 36 483580 2013-08-12 13 05 37 484931 2013-08-12 13 05 38 485505 2013-08-12 13 05 39 486945 2013-08-12 13 05 40 488386 2013-08-12 13 05 41 489819 2013-08-12 13 05 42 491202 2013-08-12 13 05 43 492486 2013-08-12 13 05 44 493865 2013-08-12 13 05 45 494987 2013-08-12 13 05 46 496479 2013-08-12 13 05 47 497824 2013-08-12 13 05 48 499286 2013-08-12 13 05 49 500232   You can see that the sub-second count is constantly increasing and thus  the start time is  drifting    This is code that correctly accounts for drift   import datetime  threading  time  next call   time time    def foo      global next call   print datetime datetime now     next call   next call 1   threading Timer  next call - time time    foo   start    foo     Its output looks like this   2013-08-12 13 21 45 292565 2013-08-12 13 21 47 293000 2013-08-12 13 21 48 293939 2013-08-12 13 21 49 293327 2013-08-12 13 21 50 293883 2013-08-12 13 21 51 293070 2013-08-12 13 21 52 293393   Here you can see that there is no longer any increase in the sub-second times    If your events are occurring really frequently you may want to run the timer in a single thread  rather than starting a new thread for each event  While accounting for drift this would look like   import datetime  threading  time  def foo        next call   time time       while True          print datetime datetime now           next call   next call 1          time sleep next call - time time     timerThread   threading Thread target foo  timerThread start     However your application will not exit normally  you ll need to kill the timer thread  If you want to exit normally when your application is done  without manually killing the thread  you should use  timerThread   threading Thread target foo  timerThread daemon   True timerThread start

User · Answer

This will insert a 10 second sleep in between every call to foo    which is approximately what you asked for should the call complete quickly     import time  while True      foo       time sleep 10    To do other things while your foo   is being called in a background thread  import time import sys import threading  def foo        sys stdout write       foo n  format time ctime      def foo target        while True          foo           time sleep 10   t   threading Thread target foo target  t daemon   True t start   print  doing other things

User · Answer

Here s a simple single threaded sleep based version that drifts  but tries to auto-correct when it detects drift   NOTE  This will only work if the following 3 reasonable assumptions are met    The time period is much larger than the execution time of the function being executed The function being executed takes approximately the same amount of time on each call The amount of drift between calls is less than a second   -  from datetime import timedelta from datetime import datetime  def exec every n seconds n f       first called datetime now       f       num calls 1     drift timedelta       time period timedelta seconds n      while 1          time sleep n-drift microseconds 1000000 0          current time   datetime now           f           num calls    1         difference   current time - first called         drift   difference - time period  num calls         print  drift   drift

User · Answer

You can execute your task in a different thread  threading Timer will let you execute a given callback once after some time has elapsed  if you want to execute your task  for example  as long as the callback returns True  this is actually what glib timeout add provides  but you might not have it installed in windows  or until you cancel it  you can use this code   import logging  threading  functools import time  logging basicConfig level logging NOTSET                      format    threadName s   message s    class PeriodicTimer object       def   init   self  interval  callback           self interval   interval           functools wraps callback          def wrapper  args    kwargs               result   callback  args    kwargs              if result                  self thread   threading Timer self interval                                                self callback                  self thread start            self callback   wrapper      def start self           self thread   threading Timer self interval  self callback          self thread start        def cancel self           self thread cancel     def foo        logging info  Doing some work          return True  timer   PeriodicTimer 1  foo  timer start    for i in range 2       time sleep 2      logging info  Doing some other work       timer cancel     Example output   Thread-1 Doing some work    Thread-2 Doing some work    MainThread Doing some other work    Thread-3 Doing some work    Thread-4 Doing some work    MainThread Doing some other work      Note  The callback isn t executed every interval execution  Interval is the time the thread waits between the callback finished the last time and the next time is called

User · Answer

Perhaps the sched module will meet your needs   Alternatively  consider using a Timer object

User · Answer

If you meant to run foo   inside a python script every 10 seconds  you can do something on these lines   import time  def foo        print  Howdy   while True      foo       time sleep 10

User · Answer

Surprised to not find a solution using a generator for timing  I just designed this one for my own purposes  This solution  single threaded  no object instantiation each period  uses generator for times  rock solid on timing down to precision of the time module  unlike several of the solutions I ve tried from stack exchange   Note  for Python 2 x  replace next g  below with g next    import time  def do every period f  args       def g tick            t   time time           while True              t    period             yield max t - time time   0      g   g tick       while True          time sleep next g           f  args   def hello s       print  hello        4f    format s time time         time sleep  3   do every 1 hello  foo    Results in  for example  hello foo  1421705487 5811  hello foo  1421705488 5811  hello foo  1421705489 5809  hello foo  1421705490 5830  hello foo  1421705491 5803  hello foo  1421705492 5808  hello foo  1421705493 5811  hello foo  1421705494 5811  hello foo  1421705495 5810  hello foo  1421705496 5811  hello foo  1421705497 5810  hello foo  1421705498 5810  hello foo  1421705499 5809  hello foo  1421705500 5811  hello foo  1421705501 5811  hello foo  1421705502 5811  hello foo  1421705503 5810   Note that this example includes a simulation of the cpu doing something else for  3 seconds each period  If you changed it to be random each time it wouldn t matter  The max in the yield line serves to protect sleep from negative numbers in case the function being called takes longer than the period specified  In that case it would execute immediately and make up the lost time in the timing of the next execution

User · Answer

Here s a nice implementation using the Thread class  http   g-off net software a-python-repeatable-threadingtimer-class  the code below is a little more quick and dirty   from threading import Timer from time import sleep  def hello        print  hello  world      t   Timer 3 hello      t start    t   Timer 3  hello  t start     after 3 seconds   hello  world  will be printed    timer will wake up ever 3 seconds  while we do something else while True      print  do something else      sleep 10

[python] Executing periodic actions in Python

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