Claude Code Prompts for pytest (Fixtures, Parametrize, Mocks)

Q: Will Claude use factory-boy or freezegun by default?

It picks up whichever it sees in your tests/. If empty: factories not used, freezegun not used. Tell Claude in CLAUDE.md what your test stack is so it doesn't randomize.

Q: How do I avoid Claude writing tests that depend on order?

Tell it: `Tests must be order-independent. No module-level mutable state. Every fixture is function-scoped unless explicitly justified.` Then run pytest --random-order to verify.

Q: Async tests — anyio or pytest-asyncio?

Pin it. `pytest-asyncio with asyncio_mode = auto in pyproject.toml. Never call asyncio.run inside a test.` Otherwise Claude alternates between the two.

Claude Code Prompts for pytest

Copy-paste Claude Code prompts for pytest — fixture design, parametrize, factory-boy, async tests, and avoiding test interdependence. Python 3.12 ready.

pytest's flexibility is its weakness from Claude's perspective — too many valid styles. Pinning the fixture style, the mocking library, and the parametrize policy makes Claude's output consistent across the codebase.

Prompt 1 — Parametrized test for a pure function

Write tests for app.utils.dates.business_days_between(start, end) at tests/utils/test_dates.py. Cases to cover: - Same day → 0 - One business day apart → 1 - Weekend skipped (Friday to Monday) → 1 - Across a US public holiday (Labor Day 2026) → expected number minus 1 - Reversed (end < start) → ValueError - Timezone-aware dates in non-UTC zones → result correct Constraints: - Use @pytest.mark.parametrize with the ids= kwarg so failure messages name the case (e.g. id='friday_to_monday'). - One @pytest.mark.parametrize per logical group — happy paths, edges, errors as three separate decorators or three separate test functions. - For the error case, use pytest.raises(ValueError, match='start must be...'). - No conftest changes; this is pure logic. - Read tests/utils/test_strings.py first to match the style.

Prompt 2 — Fixture design for an HTTP API integration test

Write an integration test for POST /api/projects at tests/api/test_projects.py. Fixtures needed: - async_client (function-scoped) — httpx.AsyncClient pointing at the FastAPI test app. Read tests/conftest.py — async_client already exists, reuse it. - authenticated_user (function-scoped) — creates a user, returns (user, headers_with_bearer). - session (function-scoped) — async SQLAlchemy session bound to a fresh transaction that rolls back on teardown. Tests: - test_create_project_201 - test_create_project_validation_400 (missing name) - test_create_project_duplicate_slug_409 - test_create_project_anon_401 Constraints: - No global state — every fixture function-scoped, transactional rollback. - Use factory_boy factories from tests/factories/ for any seed data. - Use httpx — never requests. - Async tests with @pytest.mark.asyncio (or rely on auto mode if configured). - One assertion per test, plus assertions on status_code.

Prompt 3 — Diagnose a flaky test

tests/services/test_notifications.py::test_email_sends_on_signup is flaky (fails ~5% of CI runs). Read the file + the conftest. Identify potential causes: - Test ordering dependency: any module-level state, shared fixture, or class-level fixture? - Real network call: anything that hits a non-localhost URL? - Time dependency: now()/timezone.now() called without freeze? - Race condition: any threading/asyncio without await/join? - Filesystem state: temp files in /tmp without cleanup? For each potential cause, cite the file:line and write 1 sentence on why it could cause flake. Then propose the minimum fix. Do NOT change the code yet — output the analysis first.

CLAUDE.md for pytest

## pytest policy - Python 3.12, pytest 8, pytest-asyncio (mode=auto), factory_boy, freezegun, pytest-randomly enabled — tests must pass in any order. - Fixtures default to function-scope. Session-scope requires a comment justifying it. - No mocks of HTTP — use respx for httpx, responses for requests. - One assert per test outside of status checks. Multi-assert tests get parametrized or split. - Test names: test___.

Frequently asked questions

Will Claude use factory-boy or freezegun by default?

It picks up whichever it sees in your tests/. If empty: factories not used, freezegun not used. Tell Claude in CLAUDE.md what your test stack is so it doesn't randomize.

How do I avoid Claude writing tests that depend on order?

Tell it: `Tests must be order-independent. No module-level mutable state. Every fixture is function-scoped unless explicitly justified.` Then run pytest --random-order to verify.

Async tests — anyio or pytest-asyncio?

Pin it. `pytest-asyncio with asyncio_mode = auto in pyproject.toml. Never call asyncio.run inside a test.` Otherwise Claude alternates between the two.

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