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Functions help organize code into reusable blocks. They make your programs easier to read, maintain, and test.

Defining a Function

def greet(name):
    """Print a greeting with the user's name."""
    
    print(f"Hello, {name}!")

Calling a Function

greet("Praveen")  # It prints Hello Praveen!

Using the return Statement

def add(num1, num2):
    """Return the sum of two numbers."""
    return num1 + num2

print(add(5,6)) # It prints 11

Example

Let's look at a program that finds the factorial of a number.

def factorial(num):
    """
    Return the factorial of a non-negative integer using recursion.
    Returns None for invalid input (e.g. negative numbers, non-integers).
    """
    if not isinstance(num, int) or num < 0:
        return None
    elif num == 0 or num == 1:
        return 1
    else:
        return num * factorial(num - 1)

# Test cases
print(factorial(5))         # Output: 120
print(factorial(0))         # Output: 1
print(factorial(-5))        # Output: None
print(factorial(1.8))       # Output: None
print(factorial("Praveen")) # Output: None