Python Exceptions: Read Errors and Know Their Types

What Is an Exception in Python?

An exception is an error that happens while your program is running. Python stops at the line that fails and tells you what went wrong. The lines before it have already run; the lines after it never run.

12345print("Start")
price = 300
people = 0
print(price / people)
print("End")

Start was printed, because line 1 ran fine. Then line 4 tried to divide by zero, and the program stopped. End never appeared.

Think of it like a smoke alarm in a kitchen. Everything is fine until the moment something burns. Then the alarm goes off, cooking stops, and the alarm tells you what the problem is.

Two kinds of errors

You met both kinds in chapter 1. Now they get proper names:

A syntax error means Python can't even read your code, so nothing runs, not even line 1:

12print("Start")
print("Hi"

The rules

• An exception stops the program at the line that fails.
• Everything before that line has already happened.
• Every exception has a type, like ZeroDivisionError, and a message, like division by zero.

Common beginner doubt: "Does an exception mean my code is wrong?" Not always. Sometimes it's a bug, like a typo in a name. But sometimes the code is fine and the input is bad: the user types abc where you asked for a number. You can't stop users from typing that. What you can do is handle the exception, so your program reacts calmly instead of crashing. That's what this chapter is about.

An exception is an error at run time: Python stops at that line, and the lines after it never run.

How to Read a Python Traceback

A traceback is the error report Python prints when an exception stops a program. It looks scary, but you read it from the bottom up: the last line says what went wrong, and the lines above say where.

Here is the full report for the division example, as Python prints it on a computer:

Traceback (most recent call last):
  File "main.py", line 3, in <module>
    print(price / people)
          ~~~~~~^~~~~~~~
ZeroDivisionError: division by zero

Think of it like a doctor's note. The last line is the diagnosis: what's wrong. The lines above are the details: where it hurts.

The parts

In this course, the output box shows only the most useful line, the last one:

123price = 300
people = 0
print(price / people)

How to read any error

1. Read the type. It tells you the kind of problem (you'll learn the common ones next).
2. Read the message. It often names the exact value that caused it.
3. Go to the line. Look at the code there.
4. Ask where the values came from. The bug may be on an earlier line, like people = 0 here.

That's the debugging habit from chapter 3: every error is a clue, not a punishment.

Common beginner doubt: "The traceback is long. Do I need all of it?" Usually not. Start at the last line. When your code calls a function, the traceback lists each call on its way down, but the last line still tells you what went wrong.

Read a traceback from the bottom: the type and message say what went wrong, the line number says where.

NameError, TypeError and ValueError

Python has many error types, but a few show up again and again. Each name tells you the kind of problem, so knowing them saves a lot of time. Here are the first three.

Think of them like warning lights in a car: one for fuel, one for the engine, one for the doors. Each light points you to a different place to look.

NameError: a name Python doesn't know

12name = "Sunny"
print(nmae)

Look closely at the message: nmae, not name. A NameError is almost always a typo, or a variable used before the line that makes it.

TypeError: the wrong kind of value

12age = 12
print("Rocky is " + age)

You saw this one in chapter 5. + can join two strings or add two numbers, but not one of each. The fix: an f-string, or str(age).

ValueError: the right kind, the wrong value

1marks = int("ninety")

int() does take a string, so the type is fine. But this particular string isn't a number. You met this in chapter 7, when a user typed a word instead of digits.

Telling TypeError and ValueError apart

• TypeError: "I can't do this with that kind of thing." A number where text should be, or the other way round.
• ValueError: "I can do this with that kind of thing, but not with this one." Text is fine for int(), but not the text "ninety".

NameError: a name Python doesn't know. TypeError: the wrong kind of value. ValueError: the right kind, but a bad value.

IndexError, KeyError and ZeroDivisionError

Three more error types complete the list you'll meet most. Two of them come from collections (lists and dictionaries), and one from maths.

Think of a row of lockers. Ask for locker 10 in a row of 3, and there's no such locker. Ask for a locker by a name that isn't on any door, and nobody can find it. Those are the next two errors.

IndexError: no item at that position

12friends = ["Sunny", "Rocky", "Lucky"]
print(friends[3])

Three items have the indexes 0, 1 and 2 (chapter 9). Index 3 is one past the end. Strings give the same error: "Hi"[5].

KeyError: no such key

12prices = {"tea": 20, "coffee": 30}
print(prices["cake"])

The message is the missing key itself. You learned two ways to avoid it in chapter 13: check with in first, or use get().

ZeroDivisionError: dividing by zero

123total = 90
count = 0
print(total // count)

Maths has no answer for "divide by zero", so Python stops. It often happens with an average of an empty list, or when a user types 0.

Avoid: trusting that a key is there

12stock = {"pen": 4}
print(stock["book"])

Good: check first

12345stock = {"pen": 4}
if "book" in stock:
    print(stock["book"])
else:
    print("No books")

Common beginner doubt: "Should I always check with if first?" For lists and dictionaries, if and get() work well. But some problems, like text that isn't a number, are hard to check with if. That's where try and except shine, in the next topic.

IndexError: no such position. KeyError: no such key. ZeroDivisionError: you divided by 0.

Practice: Exceptions

Solve these to lock in what you learned in this topic. Each one runs in your browser.

1. 150.Fix a TypeError in a Birthday MessageEasySolve
2. 151.Fix a KeyError in a Price LookupMediumSolve