Errors and Exceptions in Python

What are errors and exceptions?

When Python can’t execute your code, it stops and reports a problem.

Syntax errors happen when Python can’t even parse the code.
Exceptions happen while the program is running (runtime problems).

Syntax Error

A syntax error occurs when Python sees invalid code structure.

syntax_error.py

# Missing colon
if 10 > 5
    print("10 is greater")

Typical output:

Traceback

  File "syntax_error.py", line 2
    if 10 > 5
             ^
SyntaxError: expected ':'

Exception (runtime error)

Example: division by zero.

zero_division.py

print(10 / 0)

Traceback

Traceback (most recent call last):
  File "zero_division.py", line 1, in <module>
    print(10 / 0)
ZeroDivisionError: division by zero

How to read a traceback

A traceback tells you:

Where the error occurred (file and line number)
The call stack (which functions were called)
The exception type and message

Common exception types

NameError – variable not defined
TypeError – wrong type used (e.g., add str + int)
ValueError – correct type, invalid value (e.g., int(“abc”))
IndexError – list index out of range
KeyError – dict key missing
FileNotFoundError – file path not found

common_exceptions.py

# NameError
# print(x)
 
# TypeError
# print("age: " + 10)
 
# ValueError
# print(int("abc"))
 
# IndexError
# nums = [1, 2]
# print(nums[10])
 
# KeyError
# d = {"a": 1}
# print(d["missing"])
 
# FileNotFoundError
# open("missing.txt", "r")

sketch try/except is cheap until it fires p5.js

Two ways to read a key that might not be there. Asking forgiveness costs almost nothing when the key IS present -- setting up the try block is nearly free, and it beats a separate membership test. But raising and catching an exception is expensive, so when the key is usually missing the check wins by a lot. Measured nanoseconds per operation: present, 29.4 against 46.0; missing, 129.7 against 18.0. The right choice depends entirely on which case is rare.

In the next pages you’ll learn:

Handling exceptions with try/except
Cleaning up resources with finally and context managers
Creating custom exceptions

🧪 Try It Yourself

Exercise 1 – Catch a ValueError

Exercise 2 – Multiple Except Blocks

Exercise 3 – Raise a Custom Exception

pch.coffeeTagline

pch.coffeeCta

pch.feedbackHeading

pch.feedbackSubheading