assert

In Python, the assert keyword lets you create assertions to test whether a given condition is true. If the condition is false, then assert raises an AssertionError exception, optionally with a descriptive error message. Assertions are sanity checks into your code that help you identify bugs during development.

Python assert Keyword Examples

Here’s a quick example to illustrate how to use the assert keyword:

Python 
>>> x = 5
>>> y = 10
>>> assert x < y, "x is not less than y"

In this example, the condition x < y is true, so assert passes without raising an error. If x had been greater than or equal to y, the assert would have raised an error:

Python 
>>> x = 15
>>> assert x < y, "x is not less than y"
Traceback (most recent call last):
    ...
AssertionError: x is not less than y

Because x is greater than y, the assertion fails and raises an AssertionError exception with the specified message.

Python assert Keyword Use Cases

  • Quickly verifying assumptions made in your code during debugging
  • Ensuring that your program is working as expected during development by inserting sanity checks
  • Validating that certain conditions hold true during code testing
Python's assert: Debug and Test Your Code Like a Pro

Tutorial

Python's assert: Debug and Test Your Code Like a Pro

In this tutorial, you'll learn how to use Python's assert statement to document, debug, and test code in development. You'll learn how assertions might be disabled in production code, so you shouldn't use them to validate data. You'll also learn about a few common pitfalls of assertions in Python.

intermediate best-practices python

For additional information on related topics, take a look at the following resources:

By Leodanis Pozo Ramos • Updated Jan. 6, 2025 • Reviewed by Dan Bader
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