pytest fixtures supply reusable dependencies and @pytest.
Understanding the Concept
pytest fixtures supply reusable dependencies and @pytest.mark.parametrize runs one test across multiple inputs. These features reduce repetition without obscuring which case failed.
pytest must be installed separately and the test should be run with its test runner.
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.
import pytest
@pytest.fixture
def base_price():
return 10
@pytest.mark.parametrize('quantity, expected', [(1, 10), (2, 20), (4, 40)])
def test_order_total(base_price, quantity, expected):
assert base_price * quantity == expected
print('Save as test_orders.py and run python -m pytest')Walkthrough: What Each Part Does
- The imports make pytest available. These are standard-library building blocks unless the example explicitly names an external tool.
- The definition base_price, test_order_total 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
Sharing mutable fixtures outside intended scope or assuming a fixture runs even when no test requests it.
pytest must be installed separately and the test should be run with its test runner.
Practice Exercise
Create a test with three parameter cases and one fixture.
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
- Sharing mutable fixtures outside intended scope or assuming a fixture runs even when no test requests it.
- Writing assertions that check implementation details but fail to validate the behavior that matters to the caller.
Key Takeaways
- pytest fixtures supply reusable dependencies and @pytest.
- pytest must be installed separately and the test should be run with its test runner.
- Practice: Create a test with three parameter cases and one fixture.