What does if __name__ == “__main__”: do in Python?

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• on Mar 21, 2023

The line if __name__ == "__main__": in Python is used to check whether the current module is being run as the main program or it is being imported as a module into some other program. This is a common technique used in Python to write code that can be used both as a standalone program and as a module.

When a Python module is imported into another module, all of the code in the imported module is executed, including any function or class definitions, global variables, and other statements. However, sometimes you may want to write code in a module that should only be executed if the module is run as the main program, and not if it is imported as a module into some other program.

The __name__ variable in Python is a special variable that is automatically set by the Python interpreter. When a module is run as the main program, __name__ is set to "__main__". When a module is imported as a module into another program, __name__ is set to the name of the module.

Therefore, the line if __name__ == "__main__": allows you to specify which parts of the code should only be executed if the module is run as the main program. Any code that comes after this line will only be executed if the module is run as the main program, and not if it is imported as a module into another program.

Here’s a simple Python program that uses if __name__ == "__main__": to print a message only if the module is run as the main program:

def hello_world():
    print("Hello, World!")

if __name__ == "__main__":
    hello_world()

In this example, we define a function called hello_world() that prints the message “Hello, World!” to the console.

Then, we use the if __name__ == "__main__": statement to call the hello_world() function only if the module is being run as the main program. If the module is being imported into another program, the hello_world() function will not be called.

You can save this code to a file called hello.py, and then run it from the command line using the python command:

$ python hello.py
Hello, World!

Alternatively, you can import this module into another Python program, and the hello_world() function will not be called:

import hello

# This code does not call the hello_world() function because it was not run as the main program