Why write tests?

Tests in a software check whether it works as expected. Without tests, you might be able to say that your code works, but you will have no way to prove that it works correctly.

Additionally, it is important to remember that it can be dangerous to omit unit testing in Python because of its duck-typing nature. Unlike languages such as Haskell, type checking cannot be strictly enforced at compile time (though type-hinting helps). Unit tests, being run at runtime (although in a separate execution), are essential in Python development.

Writing tests can be a humbling experience. The tests will point out your mistakes, and you will get a chance to make an early course correction. In fact, there are some who advocate writing tests before the code itself.