Python for complete beginners · Unit 1: Getting started · Lesson 3 of 24

Lesson 3: Errors are normal – reading error messages and debugging

Every programmer sees error messages every day. Learn to read them, fix the most common ones, and debug like a professional.

⏱ About 35 minutesFor complete beginners: grades 6–8 and adultsRuns in your browser – nothing to install

Everyone gets errors

If you have tried the exercises so far, you have probably already seen Python complain about something. Good! Learning to deal with errors is one of the most valuable skills in programming, and today you'll learn how.

A mistake in a program is called a bug. Finding and fixing bugs is called debugging. There's a famous story from 1947: engineers working on an early computer at Harvard found a real moth stuck inside the machine, and taped it into their logbook as the "first actual case of bug being found". People already called faults "bugs" before then, but the story stuck – and so did the moth, which is now kept in a museum.

Remember from Lesson 1: computers are very literal. One missing bracket is enough to stop a program. That isn't because you're bad at this. It's because the computer can't guess what you meant.

Three kinds of error

1. Syntax errors. The code breaks the rules of how Python must be written (its syntax) – like a missing bracket or quote mark. Python can't even start, so nothing runs.
2. Errors while running. The code is written correctly, but Python hits a problem part way through, such as a name it doesn't know. It stops at that line. Anything printed before it still appears.
3. Logic errors. The program runs with no error message at all, but does the wrong thing – like adding when it should multiply. These are the sneakiest, because Python can't spot them. Only you can.

How to read an error message

When Python stops, it shows an error message. It can look scary at first, but it's just Python telling you three useful things. Here's a program with a spelling mistake on line 2, and the message Python gives:

1print("Line one works")
2pront("Line two has a typo")

Line one works

Python's full message is called a traceback. It looks like this:

1. Traceback (most recent call last) – just a heading meaning "here's where things went wrong". You can skip it.
2. line 2 – where Python noticed the problem. Look here first.
3. NameError – the kind of error. The name tells you what sort of mistake to look for.
4. name 'pront' isn't defined – the details, in Python's words. (Some versions of Python say is not defined; it means the same.)

In our editor, you'll see Python's own words as well as a plain-English explanation underneath the output. Try it – run this, read the message, then fix the typo and run it again:

print("Line one works")
pront("Line two has a typo")

The errors you'll meet most

SyntaxError: Python can't understand the line

A SyntaxError means the code isn't written the way Python needs. The usual causes are a missing bracket, a missing quote mark, or curly “smart quotes” pasted from a word processor.

Watch out: when a bracket is missing, Python often reports the next line, because it only realises something is wrong when it reaches the next line. So always check the line it names and the line above.

print("Hello"
print("Goodbye")

NameError: Python doesn't know that name

A NameError means you used a word Python doesn't recognise. Usually it's a spelling mistake (pirnt), a capital letter (Print), or text where you forgot the quote marks: print(Hello) makes Python look for something called Hello.

IndentationError: unexpected spaces at the start

The spaces at the start of a line are called indentation, and in Python they matter. For now, every line should start right at the left edge. A stray space at the start of a line gives an error. (In the editor here, Python may call this a SyntaxError.) You'll use indentation on purpose in Unit 3.

TypeError: things that don't go together

A TypeError means you tried to do something with the wrong kind of value – like joining text and a number with +, which you met in Lesson 2.

Logic errors: no message, wrong answer

This program is meant to show how many legs 3 spiders have. Spiders have 8 legs. It runs without any error. Is it right?

1print("Legs on 3 spiders:")
2print(3 + 8)

Legs on 3 spiders:
11

The only way to catch a logic error is to check the output. Before you run a program, work out roughly what it should print. Then compare.

A simple way to debug

1. Read the message. Find the line number and the error type.
2. Look at that line – and the line above. Check brackets, quote marks, spelling and capitals.
3. Change one thing at a time. If you change five things at once, you won't know which one fixed it.
4. Run it again. A new error is often good news: it means you fixed the first one and Python got further.
5. Still stuck? Explain your code out loud, line by line, to someone – or to a toy. Programmers call this rubber duck debugging, and it works surprisingly well.

Your turn: be a bug hunter

Each program below has at least one bug. Fix it, press Run to test, then press Check.

print("Good morning!")
print("Time for school.")

print("Ready")
print("Steady")
print("Go!")

print("Legs on 5 spiders:")
print(5 * 8)

print("Welcome")
print("to the bug hunt!")

Common mistakes when debugging

Quick quiz

Recap

• A bug is a mistake in a program; debugging is finding and fixing it. Errors are normal.
• An error message gives a line number, an error type and details.
• SyntaxError: the code breaks Python's writing rules. Check the line and the one above.
• NameError: Python doesn't know a name – check spelling, capitals and quote marks.
• Logic errors give no message, so always check your output.
• Debug in small steps: read, look, change one thing, run again.

That's the end of Unit 1 – well done! In Unit 2 you'll meet variables: a way for your programs to remember things.

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