Pytest Fundamentals for Data Engineers
Data engineering code is still software, and software that doesn't have tests is a liability. If you've ever had a production pipeline silently produce wrong numbers because a downstream transformation made an assumption about a column that changed upstream, you know the pain. Testing isn't just about "does the pipeline run" — it's about verifying that every transformation, every branch of logic, and every edge case behaves exactly as you expect.
This post lays the groundwork for testing data pipelines with pytest. I'll cover why testing matters, the fundamental pattern every test follows, how to write your first tests and assertions, how to verify that your code raises the right exceptions, and how to mock external dependencies so your tests stay fast and deterministic. By the end, you'll have a solid foundation for writing unit tests that catch regressions before they reach production.
Testing 101 for Data Pipelines
If you're new to testing or have only ever written a few scripts, the core idea is simple: a test is a small piece of code that exercises another piece of code and checks that the result is what you expected. In pytest, that looks like a function whose name starts with test_ and which contains an assert statement.
