Summary  
This chapter explains how to define and call functions in Python—covering the def keyword, snake_case naming, parameters and return values, indentation for the function body—and shows that functions can perform actions (printing) or return values.

General domain of usage  
basic programming education

In Python, a **function** is a named block of reusable code that performs a specific task. Functions act like LEGO pieces: small, reusable building blocks you can use throughout your program.

Definition

Using functions helps avoid rewriting code, reduces mistakes, and breaks large tasks into smaller steps. Each function handles one action, making programs clearer and easier to maintain. In short, functions let you reuse logic and keep your code organized.

## Defining a Function

Define the simplest function in Python.

1. Use the `def` keyword;
2. Write the **function name** using snake_case;
3. Add `():` — parameters go inside the parentheses;
4. Write the **function body** on an indented line;
5. Optionally specify a **return value**;
6. **Call** the function using its name.

Create a simple function that prints `'Hello, world!'` and call it.

# Specify the function name
# The function doesn't require any parameters, so we simply use `():`
def print_hello():
    # Function body: we have to use indentation when defining it.
    print('Hello, world!')
    # The function doesn't return anything, so we don't need to write additional code

# Call the function
print_hello()

The `print()` inside `print_hello()` is a **built-in function**. A function may perform an action without returning a value. Since `print_hello()` only prints a message and doesn’t provide output for further use, it has no return value.


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.

What Is Function?

Defining a Function

What Is Function?

Defining a Function