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Tests that catch a broken contract

Use examples and failure cases to protect meaningful behavior.

Reconciliation checks become executable tests.

Understand the idea

Choose tests that would catch a meaningful defect: a multiplied join, a lost zero, a conflicting key or a missing transaction rollback. A test that merely repeats the implementation’s calculation is weak evidence.

Keep network and clock behavior injectable. Use a temporary directory or database for I/O tests, then add a focused end-to-end fixture check that reconciles the exported result.

What to learn

• Run the stdlib unit tests before adding a dependency.
• Use pytest later for fixtures and parametrization.
• Test malformed input, duplicates, empty data and reruns.

Practice on your own data

Add a regression test for a defect you introduce in a working copy.

Practice this lesson in the Python lab →

Prerequisites

Testing & debugging · Files, CSV, JSON & paths

Sources and further reading

• pytest · getting started ↗Official documentation
• Harvard CS50P · lectures & problem sets ↗User-selected HarvardX CS50P course

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