How to schedule a function to run every 3 days after first run in python

Question

I have a function and i need to run it after 3 days using python

def hello()
print("hello world")

The script will be running,How to print it every 3 days in python

Answer 1

As @Nuts said in a comment, cron is the best option if you want to run an entire program every three days. However, if you're running a microservice or something and just want one particular method to execute every three days, you can use timers.

import threading

my_timer = None

def make_thread():
    # create timer to rerun this method in 3 days (in seconds)
    global my_timer
    my_timer = threading.Timer(259200, make_thread)
    # call hello function
    hello()

Just call make_thread() once to execute hello() the first time, and then it will call itself every three days (with a few seconds of margin for error, most likely), so long as the program remains running.

Answer 2

If you have to use python, one possible way (adapted slightly from this thread which has other good information) is to use a massive sleep statement.

#!/usr/bin/python3

import time 

def hello():
    print("Hello, World")

while True:
    hello()
    time.sleep(3*24*60*60) 

Here is a more complete code snippet. I tested it for shorter intervals (not 3 days) but it should probably work

import numpy as np 
import time
import datetime

def function_to_call_periodically():
    print("Hello World")

# specify the interval and the time (3 days and at 11h00)
# The code below assumes the interval is more than a day
desired_time_delta_in_seconds = 3*24*60*60  # 3 days
desired_time_minutes = 00
desired_time_hours = 11

# set the starting time the code will run every 3 days from this date (at the specified time)
start_date = datetime.datetime(2018, 8, 7) # year, month date (hours, minutes, seconds will all be zero)
start_stamp = start_date.timestamp()

# if the scripts gets restarted we need to figure out when the next scheduled function call is 
# so we need to nkow when we started
curr_time = datetime.datetime.now()
curr_stamp = curr_time.timestamp()
last_interval = (curr_stamp - start_stamp) // desired_time_delta_in_seconds


# Loop forever
while(True):
    # sleep so we don't use up all the servers processing power just checking the time
    time.sleep(5)

    # get the current time and see how many whole intervals have elapsed
    curr_time = datetime.datetime.now()
    curr_stamp = curr_time.timestamp()
    num_intervals = (curr_stamp - start_stamp) // desired_time_delta_in_seconds # python 3 integer division

    # if at least another interval has elapsed and it is currently the time for the next call
    # then call the function and update the interval count so we don't call it again
    if (num_intervals > last_interval) and (curr_time.hour >= desired_time_hours) and (curr_time.minute >= desired_time_minutes):
        print("Calling scheduled function at ", curr_time)
        last_interval = num_intervals
        function_to_call_periodically()