Basically, when you compile a program written in Rust or C/C++ (the first and second panels respectively), the compiler needs to know what’s supposed to be executed first when the program is run directly (i.e. when you click on the executable), which in these languages, is denoted by a special function called main(). Executable files can also contain functions and data structures that can be called by other programs, and when they are, you wouldn’t want to run an entire complex and resource intensive program if another program only needs to call a single function from it. In that case, the other program will call the function it wants but not main, so only that function executes and not the entire program.

However, Python is a scripting language that’s interpreted. So every Python source file is executable provided you have the Python runtime. Python also doesn’t have native support for main functions in the same way Rust and C/C++ does, and it will execute every line of code as it reads the source file. This is why a single line Python file that just calls print is valid, it doesn’t need to be wrapped in a main function to execute. However, what if your Python file is both meant to be executed directly and provides functions that other Python files can call? If you just put the main routine in the root of the file, it would be executed every time another program tries to import the file in order to call functions from it, since the import causes the file to be interpreted and executed in its entirety. You can still just have a main function in your file, but since Python doesn’t natively support it, your main function won’t do anything if you run the file directly because as far as Python is concerned, there is no executable code at the root of the file and you haven’t called any functions.

The workaround is to have a single if statement at the root of the file that looks like this:

if __name__ == '__main__':
    main()

It checks a special variable called __name__. If the Python file is directly executed, __name__ will have the value of the string '__main__', which satisfies the if statement so main() is called. If another Python file imports it, the value of __name__ will be the name of that file, so main() isn’t called. It’s clunky and not that efficient, but, 1, it works, and 2, if you cared about efficiency, you wouldn’t be writing it in Python.

Python has a bunch of magic variables, like __name__. This one contains the name of the module you’re currently in (usually based on the file name), so if your file is called foo.py, it will have the value foo.

But that’s only if your module is being imported by another module. If it’s executed directly (e.g. python foo.py), it will instead have a __name__ of __main__. This is often used to add a standalone CLI section to modules - e.g. the module usually only defines functions that can be imported, but when executed it runs an example of those functions.