Lesson 21 of 32

7 mins read

Lists & Mutable Sequences

In Plain English

A Python list is like your grocery list scribbled on paper: items have a defined order, you can add more items at the bottom with .append(), cross items off, or swap things out at will. Defined with square brackets [].

The Mental Model

"Imagine an organized spice rack where each jar has a slot number. You can add new spices to the rack, swap paprika for garlic, or count how many spices you have in total."

Deep Dive: How It Works

Lists are ordered, mutable (changeable) sequences that can store any mix of data types (integers, strings, floats, booleans, and even nested lists).

Creating lists: Defined with square brackets: fruits = ["apple", "banana", "cherry"]. An empty list is created with [] or list().

Accessing elements: Lists are 0-indexed. fruits[0] gives "apple", fruits[1] gives "banana". Negative indices count backward from the end: fruits[-1] gives the last item ("cherry").

Common methods: .append(item) adds to the end, .insert(index, item) inserts at a specific position, .remove(item) deletes by value, and len(list) returns the total item count.

Syntax Blueprint

items = ["A", "B", "C"]
items.append("D")       # Adds "D" to the end
first = items[0]        # "A"
last = items[-1]        # "D"

Use square brackets [] for creation and indexing. Indices start at 0.

Core Rules to Remember

Square Brackets []: Lists are written inside square brackets separated by commas.

Fully Mutable: You can modify, append, sort, and remove list items at any time.

Zero-Indexed: The first item is always at index 0, the last item is at index -1.

Common Beginner Traps & How to Fix Them

Mistake:IndexError: list index out of range

Why it happens: Trying to access index 3 in a 3-item list (which only has indices 0, 1, 2).

How to fix: Remember that the last index is always len(my_list) - 1 or use my_list[-1].

Pro Tip: Check if an item exists in a list instantly with the "in" keyword: if "apple" in fruits:.

Live Interactive Example

Hit Run Code to see it live

Building and Modifying a List

Python 3.12

Output Console

Click "Run Code" to view the rendered output.

How it works: len() returns the element count, while .append() dynamically attaches a new item to the end.

Your Turn: Micro Challenge

No pressure! Edit the starter code below and test your solution with instant feedback.

Micro Exercise

Append an element to a list

Create a list colors = ["red", "blue"].
Append "green" to it and print the updated colors list.

Sandbox Output

Click "Run & Check" to test your solution.

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