test-coverage-review

name

test-coverage-review

description

Audit, plan, write, and verify unit tests for Python projects using pytest. Use this skill whenever someone asks to review test coverage, write tests, add missing tests, check if code is properly tested, improve test quality, or ensure new features have tests. Trigger on phrases like "write tests for this", "add unit tests", "check my test coverage", "review my tests", "are my tests good enough", "this needs tests", "test this module", or any request involving pytest, unittest, test files, mocking, fixtures, or code coverage in a Python project. Also trigger when someone finishes writing a feature and needs tests before the work is considered complete, or when they ask about testing best practices.

Test Coverage Review

This skill audits test coverage in Python projects, identifies gaps, writes new tests, and verifies that the test suite passes — all following pytest best practices.

Before you start

Read the best-practices reference file to ground your work:

cat references/best-practices.md

Use the practices and source citations in that file as your authoritative checklist.

Modes of operation

Determine the mode from the user's request.

---

Mode 1 — Audit existing test coverage

Use when the user says "review my tests", "check my coverage", or "are my tests good enough".

Step 1: Discover the project structure

Identify:

• The package name and layout (flat or src/ layout)
• The test directory (tests/, test/, or co-located)
• The test framework in use (pytest, unittest, or both)
• Any conftest.py files and shared fixtures
• Coverage configuration in pyproject.toml, setup.cfg, or .coveragerc

Step 2: Map source modules to test files

For every public module in the package, check whether a corresponding test file exists. Flag modules with no test file at all.

Source module	Test file	Status
mypackage/api.py	tests/test_api.py	✅ exists
mypackage/parser.py	—	❌ missing

Step 3: Audit test quality

For each existing test file, check against the reference checklist:

1. Naming — File named test_<module>.py, classes prefixed Test, methods prefixed test_.
2. Docstrings — Module-level docstring, class docstring, method docstrings.
3. Structure — Tests grouped in classes by the unit under test, with section dividers between classes.
4. Isolation — No test depends on another test's state or execution order.
5. Mock boundaries — External I/O (HTTP, filesystem, database) is mocked. Internal methods are tested directly, not mocked.
6. Fixtures — Shared setup uses @pytest.fixture, not setUp() or duplicated code.
7. Sample data — Test data defined as module-level constants (not buried inside fixtures or test methods).
8. Edge cases — Empty inputs, invalid inputs, boundary conditions, and error paths are tested.
9. Assertions — Each test has clear, specific assertions. No tests that only check "does not raise".
10. Coverage — Every public method/function has at least one test. Every branch (if/else, try/except) has coverage.

Step 4: Run coverage (if possible)

pytest tests/ --cov=<package> --cov-report=term-missing --tb=short -q

Report:

• Overall coverage percentage
• Specific lines/branches not covered
• Which modules are below the project's coverage threshold

Step 5: Produce the audit report

Output a markdown table of findings:

File	Issue	Severity	Source
tests/test_api.py	No tests for error paths in fetch()	critical	[PYTEST-GOOD]
tests/test_parser.py	_parse() mocked instead of tested directly	warning	[MOCK-BOUND]
—	No test file for mypackage/utils.py	critical	[COVERAGE]

Severity levels:

• critical — Public code with no tests, untested error paths, missing test files, live API calls in tests.
• warning — Missing docstrings, over-mocking, duplicated setup code, missing edge cases.
• info — Style suggestions, parametrize opportunities, fixture improvements.

---

Mode 2 — Write tests for new code

Use when the user says "write tests for this", "add tests for this module", or has just finished a feature.

Step 1: Analyze the code under test

Read the module and identify:

• Every public class and its public methods
• Every public function
• Constructor parameters and their types
• Return types and possible exceptions
• External dependencies (HTTP calls, file I/O, database, etc.)
• Edge cases (empty inputs, None values, invalid types, boundary conditions)

Step 2: Plan test categories

For each public class or function, plan tests in these categories:

Category	What to test
Happy path	Normal inputs produce expected outputs
Edge cases	Empty inputs, None, zero, boundary values
Error handling	Invalid inputs raise expected exceptions
Return types	Return values have correct types and structure
Side effects	External calls are made with correct arguments
State changes	Object state changes correctly after method calls

Present the plan to the user before writing.

Step 3: Write the test file

Follow this structure:

"""Tests for <module description>."""

from unittest.mock import patch, MagicMock

import pytest

from <package>.<module> import <Class>, <function>


# ---------------------------------------------------------------------------
# Sample data
# ---------------------------------------------------------------------------

SAMPLE_DATA = ...  # Module-level test data constants


# ---------------------------------------------------------------------------
# <ClassName> tests
# ---------------------------------------------------------------------------


class Test<ClassName>:
    """Tests for the <ClassName> <type>."""

    def test_<behavior>(self):
        """Verify <what is being tested>."""
        # Arrange
        ...
        # Act
        ...
        # Assert
        ...

Rules:

• One test file per source module: test_<module>.py
• Group related tests in class Test<Name>: with class docstrings
• Docstring on every test method: """Verify <what>."""
• Section dividers (# ---) between test classes
• Follow Arrange-Act-Assert pattern within each test
• Sample data as module-level constants, not inside tests
• Shared setup in @pytest.fixture, not setUp()

Step 4: Mock at the boundary

Mock external dependencies only:

• requests.get / requests.post for HTTP calls
• open / pathlib.Path for file I/O
• Database connections and cursors
• Third-party API clients
• time.sleep, datetime.now when testing time-dependent logic

Do NOT mock:

• Internal methods of the class under test
• Private helper functions
• Data transformation logic
• Anything that can be tested directly with sample data

# CORRECT — mock the HTTP boundary
@patch("mypackage.api.requests.get")
def test_fetch_sends_timeout(self, mock_get):
    mock_get.return_value = MagicMock(status_code=200, json=lambda: {})
    client = APIClient()
    client.fetch("https://example.com")
    _, kwargs = mock_get.call_args
    assert kwargs["timeout"] == 10

# WRONG — don't mock internal methods
@patch.object(Parser, "_transform")  # ← test this directly instead
def test_parse(self, mock_transform):
    ...

Step 5: Run and verify

After writing tests:

# Run the new tests
pytest tests/test_<module>.py -v --tb=short

# Run the full suite to check for regressions
pytest tests/ --tb=short -q

# Check coverage of the new module
pytest tests/ --cov=<package>.<module> --cov-report=term-missing

Report the test count and pass/fail status. Note the count for the changelog entry:

- **tests/test_<module>.py**: N unit tests for <description>.

---

Mode 3 — Fill coverage gaps

Use when the user says "improve my coverage", "fill the gaps", or coverage reports show low percentages.

Step 1: Identify uncovered code

Run coverage and parse the Missing column to identify:

• Untested functions/methods
• Untested branches (if/else paths, try/except handlers)
• Untested error paths
• Dead code (code that can never be reached — flag for removal)

Step 2: Prioritize by risk

Write tests for gaps in this order:

1. Error handlers — untested except blocks, validation failures
2. Public API methods — any public method with zero tests
3. Branch coverage — untested if/else paths
4. Edge cases — boundary conditions in already-tested functions
5. Private methods — only if they contain complex logic

Step 3: Write gap-filling tests

Add tests to existing test files (don't create new files if a test file already exists for that module). Follow the same structure and conventions as the existing tests.

Step 4: Re-run coverage and report

Show before/after coverage numbers for the affected modules.

---

Constraints

• Always use pytest unless the project explicitly uses a different framework. Do not introduce unittest-style classes (TestCase, self.assertEqual) into a pytest-native project.
• Never make live API/network calls — all tests must run offline with mocks.
• Never modify source code to make it testable — if something is hard to test, note it as a finding but don't refactor the source.
• One behavior per test — each test method should verify one specific behavior. If a test has many unrelated assertions, split it.
• Tests must be deterministic — no random data, no time-dependent assertions, no ordering dependencies between tests.
• Preserve existing tests — when filling gaps, add to existing test files. Do not restructure or rename existing tests unless the user requests it.