Python Program Main Function

This article will show you the best way to handle “main” functions in Python.

Python is like a scripting language in that all lines in a Python “module” (a .py file) are executed whenever the file is run. Modules don’t need a “main” function. Consider a module named stuff.py with the following code:

def print_stuff():
  print("stuff happened!")

print_stuff()

This is the output when it is run:

$ python stuff.py
stuff happened!

The print_stuff function was called as a regular line of code, not in any function. When the module ran, this line was executed.

This will cause a problem, though, if stuff is imported by another module. Consider a second module named more_stuff.py:

import stuff

stuff.print_stuff()
print("more stuff!")

At first glance, we may expect to see two lines printed. However, running more_stuff actually prints three lines:

$ python more_stuff.py
stuff happened!
stuff happened!
more stuff!

Why did “stuff happened!” get printed twice? Well, when “import stuff” was called, the stuff module was loaded. Whenever a module is loaded, all of its code is executed. The print_stuff function was called at line 4 in the stuff module. Then, it was called again at line 3 in the more_stuff module.

So, how can we avoid this problem? Simple: check the module’s __name__. The __name__ variable (pronounced “dunder name”) is dynamically set to the module’s name. If the module is the main entry point, then __name__ will be set to “__main__”. Otherwise, if the module is simply imported, then it will be set to the module’s filename without the “.py” extension.

Let’s rewrite our modules. Here’s stuff:

def print_stuff():
  print("stuff happened!")

if __name__ == '__main__':
  print_stuff()

And here’s more_stuff:

import stuff

if __name__ == '__main__':
  stuff.print_stuff()
  print("more stuff!")

If we rerun more_stuff, then the line “stuff happened!” will print only once:

$ python more_stuff.py
stuff happened!
more stuff!

As a best programming practice, Python modules should not contain any directly-called lines. They should contain only functions, classes, and variable initializations. Anything to be executed as a “main” body should be done after a check for “if __name__ == ‘__main__'”. That way, no rogue calls are made when modules are imported by other modules. The conditional check for __name__ also makes the “main” body clear to the reader.

Some people still like to have a “main” function. That’s cool. Just do it like this:

import stuff

def main():
  stuff.print_stuff()
  print("more stuff!")

if __name__ == '__main__':
  main()

For more information, read this Stack Overflow article:
What does if __name__ == “__main__”: do?

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