Ruff statically checks Python files for many common lint issues and optionally provides safe automatic fixes.
Understanding the Concept
Ruff statically checks Python files for many common lint issues and optionally provides safe automatic fixes. Its rules are configurable through pyproject.toml; linting does not execute your application.
Example command after installation: ruff check .
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 calculate_average(values: list[float]) -> float:
if not values:
raise ValueError('nonempty list required')
return sum(values) / len(values)
print(calculate_average([3.0, 5.0]))Walkthrough: What Each Part Does
- The definition calculate_average 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
Applying all auto-fixes without reviewing a diff or confusing clean lint output with comprehensive correctness.
Example command after installation: ruff check .
Practice Exercise
Lint a Python file that contains an unused import and examine Ruff feedback.
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
- Applying all auto-fixes without reviewing a diff or confusing clean lint output with comprehensive correctness.
- Writing assertions that check implementation details but fail to validate the behavior that matters to the caller.
Key Takeaways
- Ruff statically checks Python files for many common lint issues and optionally provides safe automatic fixes.
- Example command after installation: ruff check .
- Practice: Lint a Python file that contains an unused import and examine Ruff feedback.