When to use `if __name__ == "__main__":`?

You should use if __name__ == "__main__": whenever you want a file to behave as both a reusable module and a runnable script.

Here are the three specific situations when you should use it:

1. To Prevent Code from Running During an Import

When you import a file, Python executes every line of code in that file. If you have "action" code (like print() or a function call) at the bottom of your module, it will run as soon as someone imports it.

The Problem:

# calculator.py
def add(a, b):
    return a + b

print(add(5, 5)) # I want to test this, but...

If you import calculator in another file, it will immediately print 10, which is usually unwanted behavior for a library.

The Solution:

# calculator.py
def add(a, b):
    return a + b

if __name__ == "__main__":
    print(add(5, 5)) # Now this ONLY runs if you run 'python calculator.py'

2. For Testing and Debugging

It is a great place to put "Quick Tests" for the functions inside your module. You can write your logic at the top and then write a few test cases at the bottom inside the if block.

When you are developing the module, you run the file directly to see if it works.
Once it works, you import it into your main project, and the tests stay hidden.

3. To Define a Clear "Entry Point"

In larger programs, it’s best practice to have a main() function that starts everything. This keeps your global scope clean.

def main():
    print("Starting the application...")
    # Your logic here

if __name__ == "__main__":
    main()

How it Works (The Magic)

Python automatically sets a special hidden variable called __name__ for every file:

If you run the file directly (e.g., python file.py):
Python sets __name__ = "__main__".
Result: The code inside the if block runs.
If you import the file (e.g., import file):
Python sets __name__ = "file" (the name of the file).
Result: The code inside the if block is skipped.

Summary Recommendation:

Do use it in files where you have logic/experiments at the bottom.
Do use it in the "Main" entry file of your project.
Don't need it in files that only contain function or class definitions and have no code that actually "does" anything outside of those functions.