Python main() function

In this tutorial, we will learn about the main() function in the Python programming language. And we will also understand how we can utilize the __name__ attribute in Python program in order to execute it dynamically in various contexts.

Let us begin with understanding what the main() function in Python is.

Understanding the main() function in Python

The main() is considered a unique function for several programming languages, also referred to as the point of execution for a program file. However, the interpreter of the Python programming language executes each of the lines serial-wise from the top of the file and has no explicit main() function.

Python provides other conventions in order to define the point of execution. One of them is utilizing the main() function along with the __name__ property of a python file.

Now, let us understand the __name__ property in Python programming language.

Understanding the __name__ property in Python

The __name__ property is a unique core variable in the Python that displays the name of the present module.

This property offers different values relying on where we run the Python file. Let us consider an example to understand the __name__ property in a better way.

Executing Python File as a Script

Let us consider that we have a Python file known as myworld.py containing the following content:

File: myworld.py

Output:

$ python myworld.py __main__ 

Explanation:

In the above program file, we have printed the value of the __name__ property. As a result, the value of the __name__ variable is set to __main__.

Executing Python File as a Module

We can also execute a Python file as a module. In order to perform this, we need to import the required file into another Python program. Let us consider the following example to understand the concept properly.

Suppose that we have created a Python file as python_main.py in the same directory as the myworld.py file with the following content:

File: python_main.py

Output:

$ python python_main.py myworld 

Explanation:

In the above program file, we have imported a module as myworld. As a result, when we executed the python_main.py file, the program runs the complete code in the module file. However, we can observe that the displays the module name that is myworld instead of displaying __main__.

This happens because, in the context of executing a Python file as a module, the module name itself is assigned to the __name__ variable.

Using if statement with the __name__ variable

Since we have correctly understood how the __name__ property is assigned values, we can utilize the if statement in order to run the same Python file differently in various contexts.

Let us consider the following example where we have changed the content of the myworld.py file.

File: myworld.py

Output:

$ python myworld.py This is my Python program. 

Explanation:

In the above snippet of code, we have defined a function as main() to print some strings for the users. We have then used the if conditional clause to check if the value of the __name__ variable is equal to the __main__, then the main() function will execute. As a result, when we executed the myworld.py file, the string message is printed for the user.

However, when we execute the file as a module by importing it in the python_main.py, the program returns no output as the main() function is not called.

Thus, we can conclude the custom main() function we have defined in the myworld.py file can only be executed as a standalone script but not as an imported module.

This is the standard method to define the main() function explicitly in Python. It is among the most popular use cases of the __name__ property of a Python file.