if __name__ == "__main__":do?
You may have come across this
if __name__ == "__main__": code block and wondered what it is used for and whether you can use it in your own code.
What does this line of logic do? What are these variables referring to?
To understand what
if __name__ == "__main__": does, let’s take a look at this variable:
__name__ variable is set by our Python interpreter when we run our code and it’s defined for each file.
If the file is used as the starting point of your program, then the
__name__ variable is set to
"__main__". If the file is imported, then the
__name__ variable is set to the filename without the
.py extension. For example, if you import
__name__ will be
We can use this
__name__ variable to allow a file to execute different code blocks based on whether that file is being run as the main program or as an imported module.
In a file called
file_one.py below, three functions are defined and called:
# file_one def always(): print("Always") def on_import(): print("On import") def when_main(): print("When Main") always() if __name__ == "__main__": when_main() else: on_import()
The output of the code above, when run as the main program, is this:
Always When Main
always() function is executed whether the file is run as the main program or imported as a module. The
when_main() function, however, is executed because this file is executed as the main program.
In another scenario, a different file imports the file above, like so:
# file_two import file_one if __name__ == "__main__": print("file_two is the main program")
file_two.py is run as the main program, the output will be:
Always On import file_two is the main program
on_import() function is executed in
file_one. This is because the else statement is triggered in the
if __name__ == "__main__": code block, as
file_one is not the main program but is executed on import.
always() function from
file_one is also executed, as it falls outside of the if-else statement and is executed whether
file_one is the main program or imported.
file_two triggers the if statement of the
if __name__ == "__main__": code block and prints.