Python main function

Python main function

In this tutorial, we will learn how to use a Python program's __name__ attribute to run it dynamically in different contexts.

What is the main() function in Python?

Some programming languages have a special function called main() which is the execution point for a program file. Python interpreter, however, runs each line serially from the top of the file and has no explicit main() function.

Python offers other conventions to define the execution point. One of them is using the main() function and the __name__ property of a python file.


What is __name__ in Python?

The __name__ variable is a special builtin Python variable that shows the name of the current module.

It has different values depending on where we execute the Python file. Let's look at an example.


Running Python File as a Script

Suppose we have a Python file called helloworld.py with the following content:

print(__name__)

If we run helloworld.py from the command line, then it runs as a Python script. We can run the Python program using the following command:

python helloworld.py

When we run the program as a script, the value of the variable __name__ is set to __main__. So the output of the following program will be:

__main__

Running Python file as a Module

We can also run a Python file as a module. For this, we have to import this file into another Python program. Let's look at an example.

Suppose we have a Python file called main.py in the same directory as the heloworld.py file. It has the following content:

import helloworld

When we run this file, we will have the following output:

helloworld

Here, we can see that importing a module also runs all the code in the module file.

But, we can see that instead of displaying __main__, the program displays the name of the module i.e. helloworld.

It is because, in the context of running a Python file as a module, the name of the module itself is assigned to the __name__ variable.


Using if conditional with __name__

Now that we have understood how __name__ variable is assigned values, we can use the if conditional clause to run the same Python file differently in different contexts.

Let's look at an example.

Suppose we change the content of the helloworld.py file to the following:

def main():
    print("Hello World")

if __name__=="__main__":
    main()

Now, when we run it as a script via the command line, the output will be:

Hello World

However, when we run it as a module by importing it in the main.py file, no output is displayed since the main() function is not called.

Here, we have created a custom main() function in the helloworld.py file. It is executed only when the program is run as a standalone script and not as an imported module.

This is the standard way to explicitly define the main() function in Python. It is one of the most popular use cases of __name__ variable of a Python file.