FastAPI testing snippets

TL;DR

• Use TestClient for sync tests, httpx.AsyncClient for async.
• Override dependencies with app.dependency_overrides — don't monkeypatch.
• Build a single client fixture and let every test reuse it.
• Test the boundary, not the framework.

The base fixture

import pytest
from fastapi.testclient import TestClient
from app.main import app
from app.deps import get_db
 
@pytest.fixture
def client(db_session):
    app.dependency_overrides[get_db] = lambda: db_session
    with TestClient(app) as c:
        yield c
    app.dependency_overrides.clear()

Two things going on:

1. The DB dependency is replaced with the test session (rolled back per test).
2. The lifespan runs because TestClient is used as a context manager — startup hooks fire, shutdown hooks fire.

Async tests with httpx

import pytest
import httpx
from app.main import app
 
@pytest.mark.asyncio
async def test_stream_endpoint():
    transport = httpx.ASGITransport(app=app)
    async with httpx.AsyncClient(transport=transport, base_url="http://t") as ac:
        r = await ac.get("/v1/stream")
    assert r.status_code == 200

Use this when you have streaming endpoints, websocket handshakes, or just want to assert on async I/O without TestClient's sync wrapper.

What to actually test

• The request/response contract. Schema in, schema out, the right status codes for each.
• Auth and authorization. Every protected endpoint has at least one "no token = 401" test.
• The unhappy paths. Validation errors, business rule violations, missing resources.

What I don't test:

• That FastAPI parses a path parameter. The framework already tests that.
• That Pydantic rejects a bad shape. Same.

The boundary

A FastAPI test that calls all the way down to the database is an integration test, and that's fine. A FastAPI test that mocks everything below the handler is testing nothing useful. Pick the boundary that matches the value of the test.