Python __name__ and if __name__ == '__main_

This guide explains __name__ and if __name__ == '__main__' through a focused Python example. You will learn the behavior to expect, why it matters in a real program, and how to recognize the common mistakes before applying it in your own project.

What You Will Learn

When a module is launched as the main program, its __name__ is __main__. When another module imports it, __name__ is its importable module name. A main guard keeps command-line behavior from running during import.

When and Why to Use It

The following example focuses on a practical use of __name__ and if __name__ == '__main__'.

Step-by-Step Example

def greet_user(name):
    return f"Hello, {name}"

if __name__ == "__main__":
    print(greet_user("Sam"))

Understanding the Example

Saving and running this file prints Hello, Sam. Importing the file allows another script to call greet_user without automatically printing that greeting.

Expected Output

Hello, Sam

Important Behavior and Edge Cases

Avoid placing interactive input and long-running work at module top level when the module will also be imported.

Try It Yourself

Create helper.py with a main guard, then import it from app.py.

Common Beginner Mistakes

Avoid placing interactive input and long-running work at module top level when the module will also be imported.
Assuming that an import has no initialization effects or that every environment resolves modules in the same way.
Skipping the practice step and assuming the example covers every input. Test at least one normal case and one edge case relevant to this topic.

Key Takeaways

When a module is launched as the main program, its __name__ is __main__.
Saving and running this file prints Hello, Sam.
Apply the technique deliberately, checking its documented input requirements and failure behavior before integrating it into a larger project.