ReddRadar
Agent skill
python3-testing
Python testing patterns with pytest, pytest-mock, hypothesis, and coverage strategy. Load when writing tests, designing fixtures, or setting up coverage. Activated on test phases, parametrization, async testing, property-based testing, and mutation testing decisions.
Install this agent skill to your Project
npx add-skill https://github.com/Jamie-BitFlight/claude_skills/tree/main/plugins/python-engineering/skills/python3-testing
SKILL.md
Testing Patterns
Consult python3-core for standing defaults (coverage, test naming, AAA).
Test Failure Mindset
Tests are specifications. When a test fails, investigate both possibilities:
| Hypothesis A | Hypothesis B |
|---|---|
| Test expectations are wrong | Implementation has a bug |
| Test is outdated | Test caught a regression |
| Test has wrong assumptions | Test found an edge case |
Red flags: Never immediately change tests to match implementation. Never assume implementation is always correct. Never bulk-update tests without individual analysis.
Fixture Design
- Session fixtures for expensive resources (DB, servers)
- Module fixtures for shared test data
- Function fixtures for isolated per-test data
- Factory pattern for complex test objects
from pathlib import Path
from string import Template
FIXTURES_DIR = Path(__file__).parent / "fixtures"
@pytest.fixture
def mock_binary(tmp_path: Path) -> Path:
template_path = FIXTURES_DIR / "binaries" / "mock_binary_template.sh"
template = Template(template_path.read_text())
content = template.substitute(binary_name="tool", version="1.0.0")
binary = tmp_path / "tool"
binary.write_text(content)
binary.chmod(0o755)
return binary
Coverage Targets
| Code Type | Minimum |
|---|---|
| Business logic | 90% |
| Standard code | 80% |
| Scripts/utilities | 70% |
| Critical paths | 95% + mutation testing |
# pyproject.toml
[tool.coverage.run]
branch = true
source = ["src"]
omit = ["**/tests/**"]
[tool.coverage.report]
fail_under = 80
exclude_lines = [
"pragma: no cover",
"if TYPE_CHECKING:",
"raise NotImplementedError",
]
Property-Based Testing
When Hypothesis is available:
- Round-trip tests for parsers/serializers
- Invariant tests for state machines
- Boundary validation with
@given(st.from_type(T))
from hypothesis import given, strategies as st
@given(st.lists(st.integers()))
def test_sort_maintains_length(data: list[int]) -> None:
"""Sorting preserves all elements."""
result = sorted(data)
assert len(result) == len(data)
Mutation Testing
For critical code (payments, auth, data validation):
uv run mutmut run --paths-to-mutate=packages/module/
uv run mutmut results
Target: >90% mutation score for critical code paths.
Test Directory Structure
tests/
├── conftest.py # Shared fixtures
├── unit/ # Fast, isolated tests
├── integration/ # Tests with external dependencies
├── e2e/ # End-to-end workflows
└── fixtures/ # Test data files
References
references/testing-standards.md— full testing standardsreferences/agent-prompts.md— agent test promptsreferences/plan-templates.md— test plan templates
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