Lists: Your First Collection

12 min 20 XP

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Lesson 1 of 6 · View course roadmap

Step 1

Learn the idea

A list stores an ordered collection of items — any types, any length, changeable at any time:

fruits = ["apple", "banana", "cherry"]
fruits[0] → "apple" (index from 0), fruits[-1] → last item
fruits.append("kiwi") — add to the end
len(fruits) — how many items
"apple" in fruits → True — membership test

Loop over a list directly — no indexes needed: for fruit in fruits:. This is the Pythonic way.

Where you'll use this

API responses arrive as lists of records; every table, feed and search result you render starts life as a list. append() inside a loop is the fundamental 'collect results' pattern of data processing.

Common mistakes

IndexError: list index out of range — a 4-item list has indexes 0–3, and an empty list has none at all.
Copy confusion: b = a makes both names point to the same list; changing one changes 'both'. Real copy: b = a.copy() or b = a[:].
append() vs extend(): append([4,5]) adds one nested list; extend([4,5]) adds two numbers.

Pro tip

Lists have a fast membership test but for thousands of 'x in collection' checks, convert to a set first — it's hundreds of times faster on large data.

Step 2

Watch how it runs — line by line

Press play to watch Python execute this code, one line at a time.

Step 3

Try it yourself

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The lesson example is loaded and ready — press Run, then change something and run it again. Breaking it is part of learning. Want a clean slate? Tap “New blank”.

PYexample.py

⌘ + Enter to run

Output appears here…

Step 4

Pass the challenge +20 XP

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Create a list langs containing "Python", "SQL" and "JavaScript". Append "Rust", then print the list's length and its last item.

Target output

4
Rust

PYchallenge.py

langs = ["Python", "SQL", "JavaScript"]
langs.append("Rust")
print(len(langs))
print(langs[-1])

Run your code to check it…

Step 5

Check your understanding

1. What is nums[-1] for nums = [4, 5, 6]? 4 5 6 An error

2. Which method adds an item to the end of a list? add() push() append() insert()

Last step

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