Testing Data Code With Pytest
Write tests for data transformations.
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Help me test my data code. A function or transformation: [FUNCTION_CODE]. Typical inputs and edge cases I know: [INPUTS_EDGE_CASES]. Write pytest tests using small hand-made DataFrames: the normal case, empty input, missing values, duplicate keys, wrong dtypes, and an idempotence check. Show how to compare DataFrames with the testing helpers in pandas, how to use fixtures and parametrisation, and how to test that errors are raised with clear messages. Add property-style checks (row counts, totals preserved) and guidance on keeping tests fast. Provide a folder layout and a command to run the tests. Add a complete small example: a function that fills missing values by group, three tests written out in full, and the expected output of the test run. Include a note on testing with randomness by fixing seeds, and on avoiding tests that depend on the order of rows. Also add a short list of the tests that give the most value if time is limited, ranked, and a note on running the tests automatically on each change.
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