In Python, the if __name__ == “__main__” idiom is used to indicate that a block of code should only be executed when the script is run directly, and not when it is imported as a module. It is often used to define the main function in a Python script.

The main function is the entry point of your program, it’s the first function that gets executed when you run the script. In a script, you can have many functions but the main function is the one that gets executed first.

The if __name__ == “__main__” idiom is a guard statement that checks if the current script is the main script. When a script is run, the interpreter assigns the special variable __name__ the value “__main__”. Therefore, when the script is run directly, __name__ will be set to “__main__” and the code following the if __name__ == “__main__”: block will be executed.

Here is an example of how to define main function in python:

def main():
    print("This is the main function")

if __name__ == "__main__":
    main()

In this example, a function called main() is defined, it contains the code that you want to execute when the script is run directly. The if __name__ == “__main__” idiom is used to make sure that the code inside the main() function is executed only when the script is run directly, and not when it is imported as a module.

Another way of defining main function is by using the function keyword def and calling the main function inside a separate function as shown below:

def main():
    print("This is the main function")

def run():
    main()
    
if __name__ == "__main__":
    run()

In this example, the main function is called inside the run() function, and the run() function is called inside the if __name__ == “__main__” block.

In python, there is no strict rule on how to define the main function, you can create it in any way that makes sense to you and your program, but the if __name__ == “__main__” idiom is the most common way to define the main function in python to make sure that the code inside the main function is only executed when the script is run directly.