A regression test records the behavior that failed before changing the implementation.
Understanding the Concept
A regression test records the behavior that failed before changing the implementation. Reproduce the bug with one focused assertion, fix the function, and keep the test so later edits cannot silently reintroduce the defect.
Run the regression test before and after a change; a test that never fails cannot demonstrate it detects the defect.
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 unittest
def normalize_code(value):
return value.strip().upper()
class CodeTests(unittest.TestCase):
def test_leading_space_regression(self):
self.assertEqual(normalize_code(" ab-12 "), "AB-12")
unittest.main(exit=False)Walkthrough: What Each Part Does
- The imports make unittest available. These are standard-library building blocks unless the example explicitly names an external tool.
- The definition normalize_code, CodeTests 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 completed in the local test environment. Its output is not fixed enough to publish as a universal expected result; inspect the result on your own computer.
Important Behavior and Edge Cases
Writing a test that merely repeats the current buggy output instead of asserting the intended behavior.
Run the regression test before and after a change; a test that never fails cannot demonstrate it detects the defect.
Practice Exercise
Add a test for a lower-case value with tabs, then remove the implementation temporarily to confirm the test fails.
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
- Writing a test that merely repeats the current buggy output instead of asserting the intended behavior.
- Writing assertions that check implementation details but fail to validate the behavior that matters to the caller.
Key Takeaways
- A regression test records the behavior that failed before changing the implementation.
- Run the regression test before and after a change; a test that never fails cannot demonstrate it detects the defect.
- Practice: Add a test for a lower-case value with tabs, then remove the implementation temporarily to confirm the test fails.