Step 1

Run
Start the interpreter using python3 and print 'Hello World' using the print() function.

Step 2

Edit
Write a python script containing the same function.

Run
Run the script python3 script.py. What happens?

Step 3

Edit
Now put the print() statement inside a function.

Run
Run the script again. What changed?

Run the script interactively using python3 -i script.py and once the interpreter has started call your function.

Step 4

Edit
In the script, call the function at the top level.

Run
Run the whole thing again.

Open the interpreter python3 in the same directory where your script resides and import (see below) the script. What happened?

import your_script_name_here  # notice no '.py' at the end

Step 5

Edit
Add the main script boilerplate code:

if __name__ == '__main__':
    your_function()

And call your function inside the if block.

Run
Run it again.

Open the interpreter again and import the script again. What changed?