What does if __name__ == "__main__" do in Python?

One of the most common lines of code you'll see in Python goes something like this:

if __name__ == "__main__":
  print("Hello, World!")

While you might have seen this before, even used it a few times, it might not be clear what it does and how it works. Let's consider the following setup - two scripts, script1.py and script2.py. script2.py imports script1.py and calls a function from it.

def do_stuff:
  print('Doing stuff')

do_stuff()

from script1 import do_stuff

do_stuff()

What do you think happens when you run each of these scripts? Let's see.

$ python script1.py
# Logs: Doing stuff

$ python script2.py
# Logs: Doing stuff
# Logs: Doing stuff

As you can see, the function do_stuff is called twice when we run script2.py. This is because when we import a module, Python executes all the code in the module. So, when we import script1.py in script2.py, the function do_stuff is called once. Then, when we call do_stuff again, it's called a second time.

How do we fix this? By adding the if __name__ == "__main__" check in script1.py. This directive checks if the script is being run directly, or if it's being imported. If it's being run directly, the code inside the if block is executed, otherwise it's not.

def do_stuff:
  print('Doing stuff')

if __name__ == "__main__":
  do_stuff()

Now, when we run script1.py directly, the function do_stuff is called once. When we import script1.py in script2.py, the function do_stuff is not called. So, when we call do_stuff in script2.py, it's called only once.

$ python script1.py
# Logs: Doing stuff

$ python script2.py
# Logs: Doing stuff