Django Test Performance

Django Test Performance

Use this as the orchestration skill for speeding up a Django test suite. Do not start by changing settings blindly; first measure the suite and then pick the smallest safe optimization with a repeatable verification command.

Core Workflow

1. Establish a baseline.

   • Run the same command developers or CI use.
   • Capture wall-clock time, runner, settings module, database backend, worker count, and branch.
   • If the cause is unclear, use django-test-profiling before changing behavior.

2. Classify the dominant cost.

   • Startup/import/discovery: reduce collection scope and expensive app initialization.
   • Database creation/migrations: reuse DB locally, squash migration history, or improve database storage.
   • Per-test setup: reduce fixture data, use factories, and use setUpTestData.
   • External backends: replace file storage, cache, task queues, and instrumentation with test-safe backends.
   • Long tail of tests: parallelize after isolation checks.

3. Apply low-risk wins first.

   • Fast password hasher.
   • DEBUG = False in tests.
   • Disable database serialization when serialized rollback is not needed.
   • Disable debug toolbar, Sentry/Rollbar/APM initialization, and other instrumentation.
   • In-memory or local test backends for storage, cache, and task queues.
   • See speed-wins.md.

4. Improve data and structure.

   • Use django-test-data for factory, fixture, setUpTestData, TestCase, and integration-vs-unit decisions.

5. Improve migrations and database setup.

   • Use --keepdb or --reuse-db for local repeat runs.
   • Force rebuilds when migration history changes.
   • Prefer squashing migrations over disabling migrations globally.
   • Keep the test database backend close to production unless the project is database-agnostic.

6. Parallelize only when safe.

   • Use django-test-parallelization for order isolation, shared resources, and worker configuration.

7. Re-measure with the original baseline command.

   • Report before/after runtime and the changed risk surface.
   • Keep or revert each optimization based on measured impact and behavioral fidelity.

Default Commands

time python manage.py test
python manage.py test --timing
python manage.py test --keepdb

time pytest
pytest --durations 20
pytest --reuse-db
pytest --create-db

Decision Rules

• If you cannot name the bottleneck, profile first.
• If a setting changes production-like behavior, keep the override narrow and document why tests remain representative.
• Avoid broad TESTING = True branches in application code; use explicit settings and test helpers.
• Do not swap PostgreSQL/MySQL/MariaDB projects to SQLite just for speed unless production is SQLite or the project intentionally supports many backends.
• Do not disable migrations as the normal path. If a legacy project insists, run real migrations in CI.
• Keep local speed shortcuts from hiding CI coverage. CI should run slow tests and migration-realistic paths somewhere.

Handoff Map

Verification

Before finishing, provide:

• Baseline and final runtime from the same command.
• A ranked list of changes made or recommended.
• Risks introduced by each test-only override.
• Commands the project should keep for local and CI verification.