How threading.timer works in python [duplicate]

Question

I want to run a function every n seconds. After some research, I figured out this code:

import threading

def print_hello():
    threading.Timer(5.0, print_hello).start()
    print("hello")

print_hello()

Will a new thread be created every 5 sec when print_hello() is called?

@VivekKumar - no, a new thread is created for each timer event. Timer is a subclass of Thread. — tdelaney
– tdelaney, Commented Feb 17, 2017 at 7:25
@tdelaney Ohk. I looked at docs.python.org/2/library/threading.html#threading.Timer and inferred that its a single thread which will execute repeatedly — Vivek Kumar
– Vivek Kumar, Commented Feb 17, 2017 at 8:41
@VivekKumar the threading.Timer instance is a single instance which represents a single thread. It can be made to run repetitive code, if the function it calls contains a loop. However, as explained in that documentation link, print_hello will only be called once by the threading.Timer object. Multiple threads are created because print_hello creates a new instance each time. — Karl Knechtel
– Karl Knechtel, Commented Oct 7, 2022 at 20:30

tdelaney · Accepted Answer · 2017-02-17 07:29:23Z

4

Timer is a thread. Its created when you instantiate Timer(). That thread waits the given amount of time then calls the function. Since the function creates a new timer, yes, it is called every 5 seconds.

answered Feb 17, 2017 at 7:29

tdelaney

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2 Comments

Kebman Over a year ago

Won't this fail after about 1000 iterations or so, due to Pythons recursion stack?

tdelaney Over a year ago

@kebman Not in this case. print_hello reschedules itself to be run again in the future and then returns. It didn't call itself again so it is not recursive. threading.Timer(5.0, print_hello).start() just hands the print_hello function object to the timer and says call me again in 5.

Priyank Mehta · Accepted Answer · 2017-02-17 06:40:05Z

0

Little indentation of code would help to better understand the question.

Formatted Code:

from threading import Timer

def print_hello():
    Timer(5.0,print_hello,[]).start()
    print "Hello"

print_hello()

This code works spawning a new thread every 5 sec as you are calling it recursively in every new thread call.

answered Feb 17, 2017 at 6:40

Priyank Mehta

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5 Comments

Praniti Gupta Over a year ago

Thanks for indentation. I am not sure how stackoverflow works and if I can ask follow- up questions. But, Can I achieve the same without creating new threads?

Priyank Mehta Over a year ago

Yes.. You can continue here in the comments or edit the original post question citing: PS: OK I got this..but now m stuck here..

Priyank Mehta Over a year ago

And you can always upvote the response which answers your question

Praniti Gupta Over a year ago

Okay then! So is there a way to run a function periodically without creating new threads?

Priyank Mehta Over a year ago

For parallelism you can use thread pools (group of pre-instantiated, idle threads) as described [metachris.com/2016/04/python-threadpool/] here. But for periodic execution timer threads are way to go

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