Python Main Method

Introduction to Python Main function

In general, all the programming languages have main() function as a pre-defined function/ method to indicate to the compiler that it is the beginning of the execution flow. Python language is an exception to this, as its execution is in a serial manner and the first line of the code will be the starting point by default. Despite of main() function not being a mandatory element, if the programmer wishes to have a main method to sort the other parts of the code, one can always create a user defined method, which will operate similar to other methods in python.

How to declare the Python Main Function?

The main method is defined in a similar fashion like other functions defined in python.

Basic main function:

print('Introduction to main() function')
def main():
print("This is main function")
print('Outside main function')

Output: demo.py

>>>

=== RESTART: C:/Users/hp/AppData/Local/Programs/Python/Python37-32/demo.py ===

Introduction to main() function

Outside main function

>>>

Explanation:

Python interpreter starts executing a python file from the top. First, it will print the first print statement i.e Introduction to main() function. Then it finds the main() method definition since it’s is a mere definition and not a function call so it bypasses this and executes the next print statement that follows this.

 Note:  In python, it doesn’t necessarily name ‘main’ method as main(). It could be defined with any valid identifier name like main1(), main(), etc.

Execution of main() requires a brief understanding of the __name__ variable.

Brief about __name__ variable:

__name__ variable is a special implicit variable that contains a string, and the value of the string depends on how the code is being executed.

Basically there are two ways in which python interpreters execute code and __name__ value is populated by this.

1. The most common way is to execute the file as a python script

In this case __name__ will contain the string “__main__”

2. By importing the necessary code from one python file to another file.

In this case __name__ will contain the imported module name

__name__ variable helps to check if the python file is being run directly or if it has been imported from some other file.

Example of Python Main Method

 1. Sum function: File is saved as sum_numbers.py

Python file: sum_numbers.py

def sum(a,b):
return (a+b)
print(sum(3,6))
print(“__name variable set to “,__name__)

Above program is the main program and is not imported from other python files, so the value of __name__ variable is set to __main__ as shown in the output below:

Output: sum_numbers.py

2. When python file is imported from another file

This file is saved as sum.py

Python file: sum.py

import sum_numbers
print(sum_numbers.sum(10.25,9.05))
print("value of __name__:",__name__)

Explanation:

• Sum_numbers file is being imported in program file ‘sum.py’ so all the statements inside this are executed first.

i.e print(sum(3,6)) à  9

print(“__name variable set to “,__name__) à since this is being imported from sum_numbers so __name__ is set to sum_numbers

• print(sum_numbers.sum(10.25,9.05)) à3

print(“value of __name__:”,__name__) à sum.py  is main file here so __name__  is set to __main__.

Output: sum.py

To avoid the statements to get executed from imported file ‘sum_numbers’ like in this case, we can use if conditional statements.

This can be illustrated by below code example:

The above sum_numbers.py python file can be changed as below:

Python file: sum_numbers.py

def sum(a,b):
return (a+b)
if __name__=='__main__':
print(sum(3,6))
print("__name variable set to ",__name__)

Python file: sum.py

import sum_numbers
print(sum_numbers.sum(10.25,9.05))
print("value of __name__:",__name__)

In this case output from imported file ‘sum_numbers.py’ is not printed when we execute sum.py because of this condition:

if __name__==’__main__’:

as this condition fails because sum_numbers is the imported module and not the main file.

Output: sum.py

Key points to consider when main() is being used in the python program:

1. As python is an object-oriented programming language, we should incorporate its benefits in our program. This can be done by placing bulk logic codes in compact functions and/or classes for better code reusability, better debugging, a better understanding of the program and overall code optimization.

This approach will enable us to control the execution of our code rather than letting the python interpreter execute it.

This is best illustrated in the above code example, where we used if condition to prevent the output from the imported file.

2. Use __name__ variable to control the execution of code

If __name__==’__main__:
<logic of program>

3. Create main() function and put your logic inside this

Python file: demo.py

print("Main function illustration")
def sum(a,b):
return (a+b)
def main():
print("Inside main() function")
x=int(input("enter value x"))
y=int((input("enter value y")))
print("sum of values entered is",end=' ')
print(sum(x,y))
if __name__ == "__main__":
main()

• Here we have defined sum() and main().
• Sum() will calculate the sum of two numbers,
• Inside the main() method we have prompted the user to enter values for x and y through input() statement which is then converted to integer values as input() returns string values.
• Then sum(x,y) is called inside main function and control will transfer to sum() method defined at the top which will return the sum of the numbers entered by the user.
• All the above functionalities is handled by the __name__ variable.
• As demo.py is the main program file and is not being imported from any other python file so __name__ is set to string __main__.
• If there have been some import statements then the below condition

If __name__ == “__main__”:

will fail as __name__ now will have the imported module name

In this way, I put the code which I wanted to run inside main(), programming logic in “SUM” which is being called from main() and called the main() within the conditional statement.

Output: demo.py

Conclusion – Python Main Method

Well, it’s not necessary to use the main() method in python and its completely up to the programmer whether to have it in the code or not.

However, it is good practice to use the main() method because with the help if the main() function, we can execute a ton of functionalities as and when needed, and also control the flow of execution.

Recommended Article

This is a guide to Python Main Method. Here we discuss how to declare the Python Main Function along with the Examples and Outputs. You may also look at the following article to learn more –

1. Python Built-in Functions
2. Arrays in Python
3. Sorting in Python
4. Inheritance in Python