How I structure conftest.py files across a large monorepo — the pattern that works

After three rewrites, here's the conftest.py structure that finally works for our monorepo (4 services, ~800 tests): ``` tests/ conftest.py ← global: env vars, logging, session DB fixtures/ auth.py ← auth tokens, user factories db.py ← database session, transaction rollback http.py ← httpx client, base URLs services/ billing/ conftest.py ← billing-specific fixtures only inventory/ conftest.py ← inventory-specific fixtures only ``` **Key rules we enforce:** 1. Global conftest only has fixtures needed by 3+ services 2. Each service conftest imports from `tests/fixtures/` — never from sibling service conftests 3. Fixtures that create data always clean up via `yield` + teardown 4. No business logic in fixtures — they set up state, not assert on it Before this: 1 root conftest with 60 fixtures nobody understood. After: onboarding takes hours, not days.