4. Functions

Functions let you name a reusable block of code and call it whenever you need that behavior.

• 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 itself

Chapter Video

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

Python Tutorial for Beginners 8: Functions (21:47) by Corey Schafer

Hosted on YouTube; property of its original creator, embedded here for educational reference only.

Learning Objectives

By the end of this chapter, you will be able to:

1. Define and call custom functions with def

2. Use parameters, arguments, defaults, *args, and **kwargs

3. Use return statements to send results back to the caller

4. Explain local and global scope, function composition, pure functions and side effects, docstrings, and basic lambda expressions

5. Write closures with nested functions, enclosing scope, and nonlocal

6. Trace recursive function calls using base cases and recursive cases

Chapter Flow

1. Functions

2. Function Design

3. Recursion

Glossary

#	Term	Explanation
1	Function	A named, reusable block of code that performs a task.
2	Function definition	Code that creates a function using def, a name, parameters, and a body.
3	Function call	An expression that runs a function by using its name followed by parentheses.
4	Function body	The indented block of statements that runs when a function is called.
5	Parameter	A variable name listed in a function definition.
6	Argument	A value passed into a function call.
7	Default parameter	A parameter that has a fallback value when the caller does not provide an argument.
8	*args	Syntax that collects extra positional arguments into a tuple.
9	**kwargs	Syntax that collects extra keyword arguments into a dictionary.
10	Return value	The result a function sends back to the caller.
11	Return statement	A statement that exits a function and optionally sends a value back.
12	Type annotation	A note in code that records the expected type of a parameter or return value.
13	Scope	The region of a program where a name can be used.
14	Local variable	A variable created inside a function and available only within that function call.
15	Function composition	Building a result by using one function’s output as another function’s input.
16	Docstring	A string that documents what a function does.
17	Lambda function	A small anonymous function written as a single expression.
18	Recursion	A pattern where a function calls itself.
19	Stack diagram	A diagram that shows active function calls and their local variables.
20	Infinite recursion	Recursion that never reaches a stopping condition.
21	Pure function	A function whose result depends only on its inputs and that does not change outside state.
22	Side effect	A change or interaction outside a function’s return value, such as printing or mutating a list.
23	Base case	The branch of a recursive function that stops the recursion.
24	Recursive case	The branch of a recursive function that calls the function again on a smaller or simpler input.
25	Memoization	Storing previous results so repeated function calls can reuse them.
26	Nested function	A function defined inside another function.
27	Enclosing scope	The local scope of an outer function, which a nested function can read.
28	Closure	A function that remembers values from the scope where it was created, even after that scope has finished.
29	Function factory	A function that builds and returns new, configured functions.
30	nonlocal	A keyword that lets a nested function assign to a variable in its enclosing scope.
31	Global variable	A variable defined at the top level of a program, outside any function, that code throughout the module can read.
32	global	A keyword that lets a function assign to a global variable instead of creating a new local one.
33	Keyword argument	An argument passed by parameter name, such as greet(name="Ana"), so its position does not matter.
34	Boolean function	A function that returns True or False, often used as the condition of an if statement.
35	Dead code	Code that can never run, such as a statement that comes after a return on every path through the function.
36	Stack frame	The record of one active function call and its local variables; a stack diagram shows one frame per call.

Chapter Overview Slides