You’ve started reading, running, and even writing Python scripts. Maybe you are so smart that you understand 50-60% of the code you’re looking at. That’s great! At the very bottom of many scripts, though, lurks the cruftiest of code blocks:

if __name__ == '__main__':
    main()

Wow, seriously? Double underscores? Can it get any uglier or more obscure? Not to worry; this post will move that gibberish from “WTF”-land to “Oh, of course”-ville.

The short version

That weird and off-putting code basically says “If somebody is running this program from the command line, just execute the main() function.” Put it in any script you’ll want to run as a standalone utility. (Of course, your script also needs a main() function for this to work.)

The long version

Every .py file is a module. When you run a program with a command like python my_program.py, the Python interpreter considers my_program to be the “main” module, and will signify this by setting the module’s __name__ variable to have the value '__main__' (that’s a string).

This variable starts and ends with double underscores, which marks it as a “magic” attribute (see here). You can think of this as a meta-variable – it’s not part of your code, it’s about your code.

The alternate possibility is that the interpreter is reading a module that isn’t the main module – for example, when it reads a module that’s imported by the main module. In these cases, the __name__ variable is set to the module name itself. So the if statement will be False, and the main() function will never execute.

A quick example

To see __name__ in action, create a tiny module with a main() function that just prints the module’s own __name__ variable, then run it from the command line:

Next, create an equally tiny module that imports your first module and calls that same main() function:

So when you run foo.py from the command line, its __name__ is '__main__'. When you import it, its __name__ is simply foo. If all that’s too much to take in, just absorb the short version and keep coding :)