7 best Python Assertion libraries in 2025

The assertion is a very important concept in software development. It allows you to check the conditions and make sure that everything is working properly. A very basic example is when you try to divide with 0, the program will throw an error because it's not possible. When you want to start testing your code, assertions can be very useful to ensure that it works as expected. The good thing about assertions is that it does nothing when the condition is met, so it won't affect the performance of your application. There are several libraries for Python which implement assertions and we have listed the best ones here. sure - idiomatic assertion toolkit with humanfriendly failure messages; assertpy - Simple assertion library for unit testing; expects - extensible TDD/BDD assertion library.