66 lines
2.5 KiB
Markdown
66 lines
2.5 KiB
Markdown
# Test Guard — Python / pytest Patterns
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Concrete applications of the nine rules for pytest projects. Read this when reviewing or writing Python tests.
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## Rule 2: Mock boundaries in Python
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Justified mock targets:
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- HTTP clients: `httpx`, `requests`, `aiohttp` (or use `respx` / `responses` instead of raw mocks)
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- LLM SDK calls: `openai`, `anthropic`, `litellm.completion` and friends
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- Database sessions, when the database is not the subject (see Rule 9)
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- Filesystem I/O on external paths (`tmp_path` fixture is often better than mocking)
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- Clock and randomness: `time.time`, `datetime.now`, `random` (prefer `freezegun` or injected clocks)
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Unjustified mocks (common agent-generated violations):
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- `MagicMock()` standing in for a Pydantic model or dataclass — construct the real thing
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- Mocking internal utility functions to isolate a "unit"
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- Mocking `json.loads` / `json.dumps` or other stdlib pure functions
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## Rule 3: Parametrize
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```python
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# Violation: three copy-pasted tests differing by one value
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def test_slug_lowercase(): ...
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def test_slug_strips_spaces(): ...
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def test_slug_handles_unicode(): ...
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# Fix
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@pytest.mark.parametrize(
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("raw", "expected"),
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[
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("Hello World", "hello-world"),
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(" padded ", "padded"),
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("Café Menu", "cafe-menu"),
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],
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)
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def test_slugify_normalizes_input(raw, expected):
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assert slugify(raw) == expected
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```
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## Rule 8: Real Pydantic/dataclass instances
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```python
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# Wrong — hides field typos and validation errors
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state = MagicMock()
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state.user_id = "123"
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state.status = "ACTIVE"
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# Right — Pydantic validates the construction itself
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state = UserState(user_id="123", status="ACTIVE")
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```
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If a model needs many fields, add a factory fixture or use `factory_boy` — don't fall back to `MagicMock`.
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## Rule 9: Real database via fixtures
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Use a fixture that applies real migrations (e.g., a session-scoped test database with `alembic upgrade head`), and function-scoped transactions rolled back per test. `pytest-postgresql`, `testcontainers`, or an SQLite-compatible fallback all work; the point is real schema, not a mocked session, whenever query or persistence logic is the subject.
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## pytest-specific smells
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- `assert mock.call_count == N` on anything internal — Rule 1 violation
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- `@patch` stacks three or more deep — the test is coupled to implementation; restructure
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- Asserting log output via `caplog` for messages no caller parses — Rule 4 violation
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- Fixtures that build mocks of project classes — Rule 8 violation, make the fixture build real objects
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