An Introduction to Python Exceptions

Python Exceptions: An Introduction

by Said van de Klundert Jan 29, 2024 basics python
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Watch Now This tutorial has a related video course created by the Real Python team. Watch it together with the written tutorial to deepen your understanding: Raising and Handling Python Exceptions

A Python program terminates as soon as it encounters an error. In Python, an error can be a syntax error or an exception. In this tutorial, you’ll see what an exception is and how it differs from a syntax error. After that, you’ll learn about raising exceptions and making assertions. Then, you’ll get to know all the exception-related keywords that you can use in a try … except block to fine-tune how you can work with Python exceptions.

In this tutorial, you’ll learn how to:

  • Raise an exception in Python with raise
  • Debug and test your code with assert
  • Handle exceptions with try and except
  • Fine-tune your exception handling with else and finally

You’ll get to know these keywords by walking through a practical example of handling a platform-related exception. Finally, you’ll also learn how to create your own custom Python exceptions.

Take the Quiz: Test your knowledge with our interactive “Python Exceptions: An Introduction” quiz. You’ll receive a score upon completion to help you track your learning progress:

An Introduction to Python Exceptions

Interactive Quiz

Python Exceptions: An Introduction

In this quiz, you'll test your understanding of Python exceptions. You'll cover the difference between syntax errors and exceptions and learn how to raise exceptions, make assertions, and use the try and except block.

Understanding Exceptions and Syntax Errors

Syntax errors occur when the parser detects an incorrect statement. Observe the following example:

Python Traceback 
>>> print(0 / 0))
  File "<stdin>", line 1
    print(0 / 0))
                ^
SyntaxError: unmatched ')'

The arrow indicates where the parser ran into the syntax error. Additionally, the error message gives you a hint about what went wrong. In this example, there was one bracket too many. Remove it and run your code again:

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

This time, you ran into an exception error. This type of error occurs whenever syntactically correct Python code results in an error. The last line of the message indicates what type of exception error you ran into.

Instead of just writing exception error, Python details what type of exception error it encountered. In this case, it was a ZeroDivisionError. Python comes with various built-in exceptions as well as the possibility to create user-defined exceptions.

Raising an Exception in Python

There are scenarios where you might want to stop your program by raising an exception if a condition occurs. You can do this with the raise keyword:

Illustration of raise statement usage

You can even complement the statement with a custom message. Assume that you’re writing a tiny toy program that expects only numbers up to 5. You can raise an error when an unwanted condition occurs:

Python low.py 
number = 10
if number > 5:
    raise Exception(f"The number should not exceed 5. ({number=})")
print(number)

In this example, you raised an Exception object and passed it an informative custom message. You built the message using an f-string and a self-documenting expression.

When you run low.py, you’ll get the following output:

Python Traceback 
Traceback (most recent call last):
  File "./low.py", line 3, in <module>
    raise Exception(f"The number should not exceed 5. ({number=})")
Exception: The number should not exceed 5. (number=10)

The program comes to a halt and displays the exception to your terminal or REPL, offering you helpful clues about what went wrong. Note that the final call to print() never executed, because Python raised the exception before it got to that line of code.

With the raise keyword, you can raise any exception object in Python and stop your program when an unwanted condition occurs.

Debugging During Development With assert

Before moving on to the most common way of working with exceptions in Python using the try … except block, you’ll take a quick look at an exception that’s a bit different than the others.

Python offers a specific exception type that you should only use when debugging your program during development. This exception is the AssertionError. The AssertionError is special because you shouldn’t ever raise it yourself using raise.

Instead, you use the assert keyword to check whether a condition is met and let Python raise the AssertionError if the condition isn’t met.

The idea of an assertion is that your program should only attempt to run if certain conditions are in place. If Python checks your assertion and finds that the condition is True, then that is excellent! The program can continue. If the condition turns out to be False, then your program raises an AssertionError exception and stops right away: