4. Functions

Functions let you name a reusable block of code and call it whenever you need that behavior. This chapter covers how to define functions, pass information with parameters, return results, design readable functions, and understand recursion as a function that calls itself.

• Functions reduce repetition by giving a name to a reusable process.

• Parameters let a function work with different inputs.

• Return values let a function produce a result for later code to use.

• Scope controls where names are available.

• Recursion solves a problem by reducing it to smaller versions of the same problem.

Video

This short overview introduces Python functions and the basic idea of defining reusable code with def.

Learning Goals

• Define and call custom functions with def

• Use parameters, arguments, defaults, *args, and **kwargs

• Use return statements to send results back to the caller

• Explain local scope, function composition, docstrings, and basic lambda expressions

• Trace recursive function calls using base cases and recursive cases

Chapter Flow

• 4.1. Defining Functions
• 4.2. Function Design
• 4.3. Recursion

Glossary

Term	Meaning
Function	A named, reusable block of code
Parameter	A variable name listed in a function definition
Argument	A value passed into a function call
Return value	The result a function sends back to the caller
Scope	The region of a program where a name can be used
Docstring	A string that documents what a function does
Lambda	A small anonymous function written as a single expression
*args	Syntax that collects extra positional arguments into a tuple
**kwargs	Syntax that collects extra keyword arguments into a dictionary
Recursion	A pattern where a function calls itself
Base case	The stopping condition in a recursive function
Recursive case	The part of a recursive function that calls itself on a smaller problem