Test definition

Enables automatic discovery and execution of verification logic by the test runner.

Unit testing: adding a new verification method to a TestCase subclass; TDD workflow: writing a failing test before implementing the feature; Regression testing: capturing a bug reproduction as an automated check.

Python unittest module, pytest test files, any test-driven development workflow.

def test_name(self):

def test_...(self):

name: identifier matching regex ^test_[a-zA-Z_][a-zA-Z0-9_]*$

• assert statements
• test fixtures (setUp/tearDown)
• mocking utilities

• def test_function(): (pytest style without self)\nasync def test_async(self): for async tests

Forgetting the test_ prefix — causes the runner to skip the method entirely with no error; omitting self in a TestCase subclass — raises TypeError at call time; using plain assert in unittest — fails silently unless pytest rewrites assertions; incorrect indentation of the method body — causes IndentationError or logic running outside the test.

def setUp(self): (test fixture), def tearDown(self): (cleanup), def test_*_parametrized(self, *args): (parameterized tests)

Coming from Java: expecting @Test annotation to mark tests → Python uses naming convention with test_ prefix instead; Coming from JavaScript: expecting it() or test() wrapper functions → Python uses plain method definitions with the test_ prefix; Coming from Ruby: expecting test 'description' do blocks → Python uses def with self parameter.

• unit test definition
• test fixture
• test assertion
• test parameterization

Avoid using this pattern for helper methods that don't contain assertions — prefix those with _ instead of test_. No significant performance impact; test discovery overhead is negligible. Boundary condition: pytest discovers both standalone test_ functions and test_ methods in classes, while unittest only discovers methods inside TestCase subclasses.

Makes test discoverable and explicit, enabling automated test execution and clear intent.

Like a name badge at a conference — the test_ prefix is what gets you through the door of the test runner's discovery process.

In unittest.TestCase subclasses the method must receive `self`; pytest allows omitting `self` and also supports async test methods.

Using pytest fixtures with @pytest.mark.parametrize or unittest.mock.patch for isolation.