What does __init__ == __main__ mean?

When your script is run by passing it as a command to the Python interpreter,

python myscript.py

all of the code that is at indentation level 0 gets executed. Functions and classes that are defined are, well, defined, but none of their code gets ran. Unlike other languages, there's no main()function that gets run automatically - the main() function is implicitly all the code at the top level.

In this case, the top-level code is an if block. __name__ is a built-in variable which evaluate to the name of the current module. However, if a module is being run directly (as in myscript.py above), then __name__ instead is set to the string "__main__". Thus, you can test whether your script is being run directly or being imported by something else by testing

if __name__ == "__main__":    ...

If that code is being imported into another module, the various function and class definitions will be imported, but the main() code won't get run. As a basic example, consider the following two scripts:

# file one.pydef func():    print("func() in one.py")print("top-level in one.py")if __name__ == "__main__":    print("one.py is being run directly")else:    print("one.py is being imported into another module")# file two.pyimport oneprint("top-level in two.py")one.func()if __name__ == "__main__":    print("two.py is being run directly")else:    print("two.py is being imported into another module")

Now, if you invoke the interpreter as

python one.py

The output will be

top-level in one.pyone.py is being run directly

If you run two.py instead:

python two.py

You get

top-level in one.pyone.py is being imported into another moduletop-level in two.pyfunc() in one.pytwo.py is being run directly

Thus, when module one gets loaded, its __name__ equals "one" instead of __main__.

shareedit

answered Jan 7 '09 at 4:28

Adam Rosenfield
206k49317449

 

11 

And by merely importing a script, all this script's top-level code gets executed first? –  skyork Mar 3 '12 at