Summary  
This chapter explains using None as a special return value to indicate an absent or invalid result, enabling functions to signal missing data or failed operations so callers can explicitly check for and handle those cases.

General domain of usage  
input validation

In Python, `None` represents the **absence of a value**. It is used deliberately to signal that something is missing, unavailable, or undefined, and it is different from `0`, `False`, or an empty string.

## First Case

A common real-world use of `None` is to indicate that a function searched for something but did not find it.

def find_user(users, user_id):
    for user in users:
        if user["id"] == user_id:
            return user
    return None

users = [{"id": 1, "name": "Alice"}, {"id": 2, "name": "Bob"}]
user = find_user(users, 3)

if user is None:
    print("User not found")

Here, returning `None` clearly communicates that no matching user exists, allowing the caller to handle that case explicitly.

## Second Case

`None` is also commonly used to handle invalid input without crashing the program.

def parse_int(value):
    try:
        return int(value)
    except ValueError:
        return None

result = parse_int("abc")
if result is None:
    print("Invalid number")

In this example, `None` signals that the conversion failed, making it easy to check for invalid input and respond appropriately.

Gain a solid understanding of how functions shape Python programming. Master defining and calling functions, working with arguments, handling return values, and creating flexible, efficient code using recursion and lambda expressions.

Explore what functions are and why they're essential in Python. Learn how to create functions, define arguments, handle return values, and use built-in functions effectively.

Learn how positional and optional arguments work in Python. Build adaptable functions that handle different inputs and improve flexibility in your code.

Understand how to work with arbitrary and keyword arguments to handle variable input sizes. Learn how these techniques make your functions more dynamic and organized.

Discover how return values represent a function's output. Learn to return single or multiple values, use None, and explore generators for more advanced data flow control.

Unlock advanced function concepts with recursion and lambda functions. Learn how recursion solves repetitive problems and how lambda expressions create concise, one-line functions.

None Return Value

First Case

Second Case

None Return Value

First Case

Second Case