Having established how to store data with variables and collections, and guide program execution using control flow statements, the next step is to structure code more effectively. Repeating blocks of code makes programs harder to read, maintain, and debug. Functions address this by allowing you to bundle sequences of statements into reusable units.

In this chapter, you will learn to define your own functions using the def keyword. We will cover how to pass data into functions through parameters and arguments, and how functions can send results back using the return statement. You'll also learn about variable scope, distinguishing between local and global variables, how to set default parameter values, the importance of documenting functions with docstrings, and the use of lambda expressions for creating small, anonymous functions. This allows for more modular, readable, and maintainable code.