Fastapi Testing

FastAPI Testing

Best practices for testing FastAPI applications, following the official testing docs (https://fastapi.tiangolo.com/tutorial/testing/). Covers exercising endpoints through real HTTP semantics via TestClient/AsyncClient, and swapping dependencies safely with dependency_overrides. Each rule pairs an antipattern with the idiomatic pytest pattern.

When to Apply

Reference these guidelines when:

• Writing tests for FastAPI path operations
• Swapping a dependency (DB session, auth, external client) for a test double
• Writing async tests against an ASGI app without a running server
• Reviewing or refactoring an existing FastAPI test suite

Rules

• testclient (HIGH) — test endpoints through fastapi.testclient.TestClient (real HTTP, in-process)
• dependency-overrides (HIGH) — swap deps with app.dependency_overrides, not monkeypatch
• async-client (MEDIUM) — use httpx AsyncClient + ASGITransport for async tests

How to Use

Read the individual rule file for the detailed explanation and before/after example:

rules/testclient.md
rules/dependency-overrides.md
rules/async-client.md

Each rule file contains:

• A short explanation of why it matters, tied to the official docs
• An Incorrect example (the antipattern)
• A Correct example (the official pattern)
• A link to the relevant page on https://fastapi.tiangolo.com/

All examples follow the official FastAPI documentation and avoid deprecated APIs.