Django startup time

Full background — the four mechanisms (lazy router, model registration, receiver wiring, boot GC window), the regression guards, how to add code without regressing them, how to measure, and the detailed trap write-ups: [docs/internal/django-startup-time.md](../../../docs/internal/django-startup-time.md). That doc is the single source of detail; this skill is the trigger and the checklist.

The guards live in posthog/test/test_startup_import_budget.py. When one fails, defer the import — don't remove an entry to dodge it. Conversely, when you deliberately defer a significant heavy lib off setup, add it to `FORBIDDEN_AT_SETUP` (after confirming it's absent from a bare django.setup()). New imports nobody has named yet are caught by test_no_new_heavy_imports_at_setup: any package not in posthog/test/setup_import_baseline.txt costing ≥100ms at setup fails the build — defer it; baseline only what every process genuinely needs at setup.

Defaults when adding backend code

• New app/product: register models from models/__init__.py, wire receivers from AppConfig.ready(), keep ready() light.
• New receiver: wire it at the owning AppConfig.ready(), from a dedicated light module (activity_logging.py, signals.py) — never via the API/viewset module, even if it looks light today; those accumulate heavy imports and ready() silently inherits them.
• New heavy dependency (vendor SDK, Temporal/AI/ClickHouse, pandas/pyarrow/scipy): import it function-locally on the path that uses it with # noqa: PLC0415, never at module scope.
• Schema types on a setup-path module: enums from posthog.schema_enums (cheap); pydantic models from posthog.schema only inside the method that uses them. No module-level from posthog.tasks... on setup paths — CeleryQueue lives in posthog.celery_queues.
• New viewset/route: it no longer loads at setup — don't rely on import side effects; routes go in rest_router.py, not the __init__.py shim.
• Any deferral relocates cost — ask which process pays now, on what path, and whether that path is latency-sensitive (background workers paying lazily: fine; web workers paying on first requests: usually not).

Traps to check before you commit

One line each — the doc has the full write-up and the fix recipe for every entry.

• `ready()` re-drags a heavy dep onto startup — importing a receiver's module runs its module-level imports too; re-measure after wiring.
• Silent receiver loss — lost receivers don't error; reproduce in a process that does NOT build the router (manage.py shell, celery, migrate).
• Models vanishing from the registry — a model reachable only via a viewset import breaks makemigrations/admin/mypy; import it from models/__init__.py, then dmypy stop.
• Lazy-router merge conflicts — keep the __init__.py shim, port master's aggregator deltas into rest_router.py.
• Aggregator package `__init__`s — shim with PEP 562 __getattr__, via importlib.import_module and an __all__ whitelist (catch-alls recurse or deadlock on sibling imports).
• Serializer field kwargs run at import — choices=... inputs can't be function-locally deferred; move the constant to an import-light module.
• Vanished re-exports — regenerating/shimming a module drops names other code imported from it incidentally (e.g. schema.BaseModel); grep for consumers of the old namespace, including relative-import spellings.
• Patch targets break on call-time imports — @patch("mod.helper") stops intercepting once mod imports helper at call time; patch the defining module instead.
• Snapshot regen on a bad merge — regenerate .ambr against the merged branch, then confirm it isn't masking a regression.
• Measuring the wrapper, not the work — time inside the process; importtime + tuna, not pyinstrument; grimp for cycles and door enumeration.
• Phantom importtime costs from GC — re-capture with gc.disable(); if the cost vanishes, the finding is GC, not the module. Keep the entrypoints' GC window closing in a finally.