What is the expression 'if __name__ == "__main__"' for?

Question

I notice some scripts on Python, at the very end of the code, have the following expression:

if __name__ == "__main__":
   #faça alguma coisa aqui

What’s the point of this?

Answer 1

if __name__ == "__main__" tests whether the Python script file is running as the main file or not. This is useful to avoid certain behaviors if your script is imported as a module from another script.

Within this if usually put some behaviors such as tests, output values or special functionalities.

To check how this works, try setting a file .py with only the following:

print(__name__)

Save the file and run it as:

python meuteste.py

The exit should be:

__main__

Now open the Python console and import the file. The output should be:

Python 2.7.2+ (default, Oct 15 2015, 16:17:59)
[GCC 4.6.1] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import meuteste
meuteste
>>>

Answer 2

When the interpreter Python reads a source file, it executes all the code found in it. Before executing the code, it will define some variáveis special. For example, if the interpreter Python is running this module (the source file) as the main program, it defines the special __name__ to have a value __main__. If this file is imported from another module, __name__ will be defined as the name of the other module.

source :)

Answer 3

In practical terms, all code written within that expression will only be executed if the library is used directly, for example if the file in question is executed in this way:

python ficheiro.py

The code inside the expression will be executed. Whereas if the library is imported by another module:

import ficheiro

The code inside the expression will not be executed.

Answer 4

if __main__ == '__name__': print(__main__)