What does the if __name__ == "__main__": do?

To outline the basics:

The global variable, __name__, in the module that is the entry point to your program, is ‘__main__’. Otherwise, it’s the name you import the module by.

So, code under the if block will only run if the module is the entry point to your program.

It allows the code in the module to be importable by other modules, without executing the code block beneath on import.


When the Python interpreter reads a source file, it executes all of the code found in it.

Before executing the code, it will define a few special variables. For example, if the Python interpreter is running that module (the source file) as the main program, it sets the special __name__ variable to have a value “__main__”. If this file is being imported from another module, __name__ will be set to the module’s name.

In the case of your script, let’s assume that it’s executing as the main function, e.g. you said something like

python threading_example.py
on the command line. After setting up the special variables, it will execute the import statement and load those modules. It will then evaluate the def block, creating a function object and creating a variable called myfunction that points to the function object. It will then read the if statement and see that __name__ does equal “__main__”, so it will execute the block shown there.

One reason for doing this is that sometimes you write a module (a .py file) where it can be executed directly. Alternatively, it can also be imported and used in another module. By doing the main check, you can have that code only execute when you want to run the module as a program and not have it execute when someone just wants to import your module and call your functions themselves.

See this page for some extra details.

Note (by Stainsor): If you put code before the function definitions, it will execute before main.

print("This code executes before main.")def functionA():    print("Function A")def functionB():    print("Function B")if __name__ == '__main__':    functionA()    functionB()

If this module is indeed main, this code results in:

This code executes before main.Function A Function B

If this module is not main, you get:

This code executes before main.