pytest is a third-party test runner that discovers test_* functions, supports plain assert statements and provides detailed failure introspection.
Understanding the Concept
pytest is a third-party test runner that discovers test_* functions, supports plain assert statements and provides detailed failure introspection. Install it in a virtual environment; it is not included with Python itself.
Run python -m pip install pytest before running python -m pytest.
How this fits into a real test suite
A reliable test starts with a specific expected behavior. Keep the fixture small, arrange the input, call the real code, and assert a result the reader can verify. Add one boundary or failure case so the test does not pass only for a convenient example. In a larger project, run the same test automatically after each change.
Tests are repeatable experiments. Use temporary resources and injected dependencies where possible; when the environment is part of the requirement, add a separate integration test rather than weakening the unit test.
Worked Example
Read the code from the first line to the last. Identify which values are inputs, which operation is being demonstrated and what should happen before running it.
def discounted_price(price: float, discount: float) -> float:
return price * (1 - discount)
def test_discounted_price():
assert discounted_price(100, 0.2) == 80
print(discounted_price(100, 0.2))Walkthrough: What Each Part Does
- The definition discounted_price, test_discounted_price creates reusable behavior. Its body runs when called, awaited or instantiated; defining it alone does not run the body of an ordinary function.
- The final check compares actual behavior with an explicit expectation. A successful test tells you that this particular case passed, not that every possible input works.
This demonstration focuses on one practical behavior. Before changing several lines, vary one input or one configuration choice and check whether the result changes for the reason you expected.
The example uses terminal, platform, external tool or process behavior that was not exercised automatically. Run it only in a suitable environment after reviewing the shown permissions and paths.
Important Behavior and Edge Cases
Assuming pytest is installed by default or placing test functions in files that discovery does not match.
Run python -m pip install pytest before running python -m pytest.
Practice Exercise
Save a test_*.py file and run python -m pytest.
Write down the result you expect before running your changed code. Check at least one normal case and one boundary or failure case. If your example starts workers or opens resources, verify how they are cleaned up.
Further reading: official documentation and API reference.
Common Beginner Mistakes
- Assuming pytest is installed by default or placing test functions in files that discovery does not match.
- Writing assertions that check implementation details but fail to validate the behavior that matters to the caller.
Key Takeaways
- pytest is a third-party test runner that discovers test_* functions, supports plain assert statements and provides detailed failure introspection.
- Run python -m pip install pytest before running python -m pytest.
- Practice: Save a test_*.py file and run python -m pytest.