标签：#unittest

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. The Fundamental Pattern: Arrange, Act, Assert Almost every test follows three steps: Arrange – set up the input data and any necessary context. Act – call the function or method you want to test. Assert – verify the output is correct. For a data pipeline transformation, that might be: def test_clean_amount (): # Arrange raw_data = " 1,200.50 " # what you might get from a CSV # Act result = clean_amount ( raw_data ) # Assert assert result == 1200.50 clean_amount is a pure function — no file I/O, no database calls. It just takes an input string and returns a number. That makes it straightforward to test. Assert Raises: Expecting Exceptions Data pipelines often deal with invalid data that should trigger an