Python - Lists

Overview

Estimated time: 25–35 minutes

Work with Python lists: a mutable, ordered sequence type. Learn creation, access, slicing, copying, and common methods.

Learning Objectives

• Create and manipulate lists; understand mutability.
• Use slicing and copying patterns safely.
• Apply common methods: append, extend, insert, remove, pop, sort, reverse.

Prerequisites

• Python - Variables & Data Types

Basics

nums = [1, 2, 3]
nums.append(4)
nums.extend([5, 6])
print(nums[0], nums[-1])
print(nums[1:4])

Copying (don’t alias by accident)

a = [1, 2, 3]
b = a            # alias; both refer to same list
c = a.copy()     # shallow copy

b.append(9)
print(a)  # [1, 2, 3, 9]
print(c)  # [1, 2, 3]

Sorting

items = [3, 1, 2]
items.sort()             # in-place
print(items)
print(sorted(items, reverse=True))  # new list

Common Pitfalls

• Using * to create nested lists: [[0]*3]*2 duplicates references. Use a comprehension: [[0 for _ in range(3)] for _ in range(2)].
• Modifying a list while iterating over it; iterate over a copy.

Checks for Understanding

1. What’s the difference between list.sort() and sorted(list)?
2. How do you make a shallow copy of a list?

Exercises

1. Deduplicate a list while preserving order.
2. Sort a list of dicts by a key using key= with lambda.