Errors and Exceptions

Introduction

Errors are a normal part of programming. Understanding the different types and how to read a traceback is the first step to fixing them quickly.

Types of Errors

Syntax Errors

The code is not valid Python. The interpreter catches this before running anything.

if salary > 50000
    print("High earner")
# SyntaxError: expected ':'

Runtime Errors (Exceptions)

The code is valid Python but something goes wrong when it runs.

salary = int("not a number")   # ValueError
employee = {}
print(employee["name"])        # KeyError
result = 10 / 0                # ZeroDivisionError

Common Built-in Exceptions

Exception	Caused by
ValueError	Wrong value type, e.g. int("hello")
TypeError	Wrong type, e.g. "age: " + 30
KeyError	Missing dictionary key
IndexError	List index out of range
FileNotFoundError	File does not exist
ZeroDivisionError	Dividing by zero
AttributeError	Calling a method that does not exist
NameError	Variable not defined
ImportError	Module not found

Reading a Traceback

When an unhandled exception occurs, Python prints a traceback. Read it bottom to top — the last line is the error, the lines above show where it happened.

Traceback (most recent call last):
  File "main.py", line 8, in <module>
    result = calculate(value)
  File "main.py", line 3, in calculate
    return 10 / value
ZeroDivisionError: division by zero

The error is ZeroDivisionError, it occurred in the calculate function on line 3, called from line 8 in the main script.

Practice Exercises

• Write code that deliberately causes a TypeError. Read the traceback and identify the line number.
• Cause a KeyError by accessing a key that does not exist in a dictionary. Note the key name in the error.
• Cause an IndexError by accessing an index that is out of range.
• Write a short function and call it with the wrong number of arguments. Identify the exception type.