11.7. More About main()

Here is the final copy of our main() practice program. It runs correctly.

Now let’s mess it up.

Open the random_lists.py code in the editor and scroll to the bottom. Add a function call to the code:

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def main():
   numbers = create_random_number_list(8, -10, 10)
   no_repeat_numbers = create_unique_number_list(5, 0, 10)
   letters = create_random_letter_list(8)
   numbers.sort()
   no_repeat_numbers.sort()
   print(numbers)
   print(no_repeat_numbers)
   print(letters)

main()

Run the program again. You should see extra output.

11.7.1. Why Did We Do This?

We need to demonstrate one last detail about how to correctly use a main() function.

Return to the main.py code. In this case, we want the main() function to run. However, ANY .py file can be imported as a module into any other Python program.

If we want to use the find_average_score or create_report functions in a different program, then we can easily import main.py into that other file. However, as it is written, lines 20 - 27 will ALWAYS run whenever we import the module.

We need to adjust our code so that main() executes when it is part of the central program but NOT when it is part of an imported module.

11.7.2. Wrapping Up

Before Python runs a program, it defines a few special variables. One of these is called __name__, and it is automatically assigned the string value "__main__" when the code runs by itself. For an imported module, on the other hand, __name__ matches the module name.

This means we can use __name__ to check if the code is being run by itself, or if it has been imported by another program!

In both main.py and random_lists.py, replace the function call:

main()

with this:

if __name__ == "__main__":
   main()

The program should correctly run again!

The first line uses an if statement to check the value of the __name__ variable. If the value matches "__main__", then the main() function gets called. If the condition is False, then the code was imported as a module, and we do NOT call main().

The ability to choose when to execute the main() function is extremely useful! This is especially true for code that might be imported into a different program.

Whew! That was a lot of words and code. If your eyes glazed over, be sure to pay attention to this:

Tip

  1. Avoid loose statements in your code.
  2. Include a main() function, and set it up this way:
def main():
   # Control code here...

if __name__ == "__main__":
   main()