Defining Functions in Python: A Complete Guide with Examples

Functions are one of the most important building blocks in Python programming. They allow developers to organize code into reusable, modular pieces that make programs more readable, maintainable, and efficient. In this article, we’ll explore how to define functions in Python, their syntax, types, and best practices with examples.

🔹 What is a Function in Python?

A function is a block of reusable code that performs a specific task. Instead of writing the same code multiple times, you can define a function once and call it whenever needed.

🔹 Syntax for Defining a Function

def function_name(parameters):
    """docstring (optional): describes the function"""
    # function body
    return value

• def → keyword to define a function.

• function_name → user-defined name for the function.

• parameters → optional values passed to the function.

• return → used to return a value (optional).

🔹 Example: A Simple Function

def greet(name):
    """This function greets the user by name"""
    return f"Hello, {name}!"
    
print(greet("Alice"))

Output:

Hello, Alice!

🔹 Types of Functions in Python

1. Built-in Functions

Python provides many built-in functions like len(), print(), sum(), etc.

Example:

numbers = [1, 2, 3, 4]
print(len(numbers))  # Output: 4

2. User-defined Functions

Functions created by the programmer.

Example:

def add(a, b):
    return a + b

print(add(5, 3))  # Output: 8

3. Functions with Default Parameters

def power(base, exponent=2):
    return base ** exponent

print(power(4))    # Output: 16 (default exponent = 2)
print(power(4, 3)) # Output: 64

4. Functions with Variable Arguments (*args and **kwargs)

def display(*args, **kwargs):
    print("Arguments:", args)
    print("Keyword Arguments:", kwargs)

display(10, 20, 30, name="Alice", age=25)

5. Lambda (Anonymous) Functions

Small, single-line functions using lambda.

square = lambda x: x * x
print(square(5))  # Output: 25

🔹 Return vs Print in Functions

• print() displays output on the screen.

• return sends the result back to the caller for further use.

Example:

def multiply(a, b):
    return a * b

result = multiply(4, 5)
print(result)  # Output: 20

🔹 Best Practices for Defining Functions in Python

✅ Use descriptive names for functions.
✅ Write docstrings to describe the purpose.
✅ Keep functions small and focused on one task.
✅ Use type hints for readability.

Example with Type Hints:

def divide(a: int, b: int) -> float:
    """Returns division of two numbers"""
    return a / b

🔹 Advantages of Using Functions in Python

• Code reusability – Write once, use many times.

• Modularity – Organize code into logical blocks.

• Readability – Easier to understand and maintain.

• Debugging – Simplifies error detection.

🔹 Final Thoughts

Defining functions in Python is essential for writing clean, modular, and reusable code. Whether you’re working with built-in, user-defined, or lambda functions, mastering them will help you build efficient and maintainable Python applications.