Postponing functions in python

Question

In JavaScript I am used to being able to call functions to be executed at a later time, like this

function foo() {
    alert('bar');
}

setTimeout(foo, 1000);

This does not block the execution of other code.

I do not know how to achieve something similar in Python. I can use sleep

import time
def foo():
    print('bar')

time.sleep(1)
foo()

but this will block the execution of other code. (Actually in my case blocking Python would not be a problem in itself, but I would not be able to unit test the method.)

I know threads are designed for out-of-sync execution, but I was wondering whether something easier, similar to setTimeout or setInterval exists.

Answer 1

To execute a function after a delay or to repeat a function in given number of seconds using an event-loop (no threads), you could:

Tkinter

#!/usr/bin/env python
from Tkinter import Tk

def foo():
    print("timer went off!")

def countdown(n, bps, root):
    if n == 0:
        root.destroy() # exit mainloop
    else:
        print(n)
        root.after(1000 / bps, countdown, n - 1, bps, root)  # repeat the call

root = Tk()
root.withdraw() # don't show the GUI window
root.after(4000, foo) # call foo() in 4 seconds
root.after(0, countdown, 10, 2, root)  # show that we are alive
root.mainloop()
print("done")

Output

10
9
8
7
6
5
4
3
timer went off!
2
1
done

Gtk

#!/usr/bin/env python
from gi.repository import GObject, Gtk

def foo():
    print("timer went off!")

def countdown(n): # note: a closure could have been used here instead
    if n[0] == 0:
        Gtk.main_quit() # exit mainloop
    else:
        print(n[0])
        n[0] -= 1
        return True # repeat the call

GObject.timeout_add(4000, foo) # call foo() in 4 seconds
GObject.timeout_add(500, countdown, [10])
Gtk.main()
print("done")

Output

10
9
8
7
6
5
4
timer went off!
3
2
1
done

Twisted

#!/usr/bin/env python
from twisted.internet import reactor
from twisted.internet.task import LoopingCall

def foo():
    print("timer went off!")

def countdown(n):
    if n[0] == 0:
        reactor.stop() # exit mainloop
    else:
        print(n[0])
        n[0] -= 1

reactor.callLater(4, foo) # call foo() in 4 seconds
LoopingCall(countdown, [10]).start(.5)  # repeat the call in .5 seconds
reactor.run()
print("done")

Output

10
9
8
7
6
5
4
3
timer went off!
2
1
done

Asyncio

Python 3.4 introduces new provisional API for asynchronous IO -- asyncio module:

#!/usr/bin/env python3.4
import asyncio

def foo():
    print("timer went off!")

def countdown(n):
    if n[0] == 0:
        loop.stop() # end loop.run_forever()
    else:
        print(n[0])
        n[0] -= 1

def frange(start=0, stop=None, step=1):
    while stop is None or start < stop:
        yield start
        start += step #NOTE: loss of precision over time

def call_every(loop, seconds, func, *args, now=True):
    def repeat(now=True, times=frange(loop.time() + seconds, None, seconds)):
        if now:
            func(*args)
        loop.call_at(next(times), repeat)
    repeat(now=now)

loop = asyncio.get_event_loop()
loop.call_later(4, foo) # call foo() in 4 seconds
call_every(loop, 0.5, countdown, [10]) # repeat the call every .5 seconds
loop.run_forever()
loop.close()
print("done")

Output

10
9
8
7
6
5
4
3
timer went off!
2
1
done

Note: there is a slight difference in the interface and behavior between these approaches.

Answer 2

You want a Timer object from the threading module.

from threading import Timer
from time import sleep

def foo():
    print "timer went off!"
t = Timer(4, foo)
t.start()
for i in range(11):
    print i
    sleep(.5)

If you want to repeat, here's a simple solution: instead of using Timer, just use Thread but pass it a function that works somewhat like this:

def call_delay(delay, repetitions, func, *args, **kwargs):             
    for i in range(repetitions):    
        sleep(delay)
        func(*args, *kwargs)

This won't do infinite loops because that could result in a thread that won't die and other unpleasant behavior if not done right. A more sophisticated approach might use an Event-based approach, like this one.