Testing Python code with pytest, from basics to CI

Most Python developers know they should write tests. Fewer enjoy it, usually because their tests are slow, fragile or test the wrong things. This course teaches pytest in a way that makes tests feel useful: short, fast and focused on behaviour you actually care about.

We work on a realistic codebase, a small invoicing service with business rules, a database and calls to an external payment API. You will add tests to code that has none, find real bugs on the way and refactor with confidence once the tests are in place.

What you will learn

• How pytest discovers and runs tests, and how to read its output
• Assertions, fixtures and parametrised tests
• Testing exceptions, warnings, logs and output
• Mocking external services without mocking everything
• Testing code that uses a database and a web framework
• Property based testing with Hypothesis
• Coverage, test speed and running tests in CI

A practical philosophy

Good tests describe behaviour, not implementation. They fail when something users depend on breaks, and they keep passing when you rename an internal function. Throughout the course we look at test smells, such as excessive mocking or giant fixtures, and how to fix them.

You should be comfortable writing Python functions and classes. No testing experience is required, but if you have written some tests before, the later sections will still have plenty for you.