In this blog post, I will run through 3 different ways to execute a delayed Python loop. In these examples, we will aim to run the loop once every minute.
To show the delay, we will print out the current datetime using the datetime module.
1 – Sleep
The sleep function from Python’s time module pauses the Python execution by the number of seconds inputted. The example below pauses the script for 60 seconds.
The above script has a 60 second delay between the end of a loop and the start of the next. However, what if we wanted to execute the next loop 60 seconds after the start of the previous loop.
In other words, how do we start the loop at the same time every minute.
2 – Calculate Sleep
To do this we calculate how long the loop should sleep for.
We will print the datetime every time the minutes changes. At the start of the loop we lookup the number of seconds that have passed so far this minute.
The number of seconds that have passed this minute is calculated from date_time[-2:]. Subtracting this from 60 gives the length of time in seconds for which the loop should sleep for, to execute when the next minute starts.
Once the loop has slept for the required number of seconds, we lookup the datetime again and print it out.
3 – Task Scheduler
The previous two options are good for executing a loop a few times, ten in our case. If we wanted to execute a python script continuously without expiring, we could use the above examples with an infinite appending loop.
However, if one loop errors the script will stop. Therefore, we want to execute the entire Python script once a minute using an external trigger. This is where we can use Task Scheduler.
Task Scheduler can execute a python script from source but it is often easier to use a batch file. The batch file includes the location of the python application (python.exe) and the location of the python script (.py). For more detail on using Task Scheduler and batch files to run Python scripts, please see the following datatofish post – https://datatofish.com/python-script-windows-scheduler
Our batch file is:
To demonstrate Task Scheduler, I’m going to run the following Python code every minute.
This code uses Pandas to produce a blank CSV file, but the name of CSV file is the datetime the script was run.
These following screenshots show the triggers and actions used.
This produced the following CSV files, we can see that the files takes 1 – 4 seconds to create.
In summary we have seen three different ways to delay a Python loop, two using loops inside Python and one using Task Scheduler. All can be used depending on what kind of delay is best.
You should definitely remove the 2nd example here. Please tell me you’ll never give this to a customer. A quick scan of this code shows TWO problems :
i) it’s going to go into a tight loop for roughly 10 seconds every minute, and therefore consume significant CPU resource for NO reason,
ii) Depending on what’s happening elsewhere on the machine, it’s possible that this code will not even trigger the PRINT statement every minute. It relies on the datetime.now() being called on the minute. If other resources are using the CPU at the time there’s no guarantee this code is getting called at the right time.
Thanks for your feedback.
Following your comments, I thought of a better way of executing a loop at the start of every minute and updated the post.
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