There are a number of ways that you can run a python or bash script at system start but I am about to show you possibly the easiest way, using crontab.

First you need to make that your script is executable and can run unattended. Then you need to login as the user your wish to execute the script as and edit the crontab file:

crontab -e  

If this is your first time editing the crontab file, you might be asked which editor you wish to use – I chose nano.
Once in the crontab file add a new line at the bottom of the file; it should look something like this

@reboot /the/path/to/your/script/./ [any other parameters] > /path/of/your/log.txt

Alternatively, if you don’t wish to record any output from your script you can send the output to /dev/null. Now it’s time to reboot your Debian instance and your script will be live.