Python lesson 16: Lists and indexes
US version. US grades and US spelling. · UK version of this lesson
A list stores many values in order: pets = ["cat", "dog", "owl"]. A complete 40-minute Python lesson for complete beginners (Grades 6–7), from Unit 5: Lists. It runs right here in your browser – nothing to install.
- Time
- 40 minutes + 20 min homework
- Grade
- Grades 6–7 (ages 11–12)
- You need
- A computer or tablet, paper and a pencil
- Graded
- Instantly, with explanations
For parents: how to use this lesson
You don't need to know Python yourself. The lesson explains each step on screen, the way Mr Babbage, our Head of Computing, likes it: a worked example first, then a go for your child. Code questions run your child's program in the browser and grade it straight away.
- Read what your child will learn together.
- Press Start the lesson below. Sit with your child for the teaching and the worked example (about 16 minutes): ask them to say what each line does.
- Let them do the practice on their own. When code goes wrong, the error message is a clue – let them read it out and try again before you help.
- Finish with the exit questions and the talk question: “Why do you think indexes start at 0?”
- Later that day or the next day: the homework (20 minutes), then a practice test.
Watch out for
- The first item is [1]. Print pets[0] and see.
- The last index is len(). A 3-item list has indexes 0, 1, 2.
Miss Lovelace's tip for home: ask your child to explain one line of their program to you in their own words. If they can explain it, they understand it – and every error message is a clue, not a failure.
What your child will learn
Learning goal: make a list.
Success looks like
- I can make a list.
- I can get items by index, counting from 0.
- I can use negative indexes and len().
Key words
list · index · item · IndexError
What to have ready
- A device with a web browser (the code runs in the browser – nothing to install)
- Paper for tracing
- A keyboard helps for typing code (a tablet or phone works too)
The big ideas (for the adult)
A list stores many values in order: pets = ["cat", "dog", "owl"].
Indexes start at 0: pets[0] is "cat", pets[1] is "dog". pets[-1] is the last item.
len(pets) counts the items: 3. So the last index is len(pets) - 1, and pets[3] is an IndexError.
Worked example
pets = ["chess", "tennis", "hockey"] print(len(pets))
It prints:
3
The lesson, step by step
| Part | Time | How it is checked |
|---|---|---|
| Starter | 5 min | Graded by Classfern |
| Teaching input | 8 min | Read, watch and talk |
| See it, then predict | 8 min | Graded by Classfern |
| Put it in order, write it, fix it | 14 min | Graded by Classfern |
| Closure and exit ticket | 5 min | Graded by Classfern + self-check |
The lesson
It runs here on the page. No sign-up and no names – progress is kept only in this browser. Each visit picks a new mix of questions.
Homework
20 minutes: 7 questions – predict what code prints, write or fix a short program, and check the key ideas – then a short reflection. New questions every time, all graded instantly.
Practice tests
Curriculum
CSTA K–12 Computer Science Standards (2017), grades 6–8: 2-AP-11 – Create clearly named variables of different data types and operate on them.
The standard codes are CSTA's; the descriptions are our own words.
Our lessons and questions are our own. They are not written or endorsed by any government department, testing agency or exam board. Python is a registered trademark of the Python Software Foundation; Classfern is not affiliated with or endorsed by it.