Speed up FastAPI tests 7x by session-scoping the test client fixture

[RayBB]

Speed up FastAPI tests 7x by session-scoping the test client fixture

Problem

The FastAPI test suite (openlibrary/tests/fastapi/) was significantly slower than it needed to be. Profiling revealed that the fastapi_client pytest fixture was function-scoped (the default), meaning it recreated the entire FastAPI app from scratch for every single test — including importing and registering all 13 routers, adding middlewares, mounting static files, etc.

Benchmarks showed the disparity clearly:

Test group	Tests	Time	Per test	Relative speed
Non-FastAPI tests	1,179	1.75s	~1.5ms	1×
FastAPI tests (before)	210	7.50s	~35.7ms	~24× slower

The fastapi_client fixture alone cost 0.04–0.12s per invocation, and ~113 tests (54%) depended on it either directly or through mock_authenticated_user or the client alias.

Solution

Changed the fixture scope from function (default) to session:

# Before
@pytest.fixture
def fastapi_client():
    ...

# After
@pytest.fixture(scope="session")
def fastapi_client():
    ...

The FastAPI app is stateless — it's purely route registration + middleware setup. All mutable state (dependency_overrides in mock_authenticated_user) is properly managed with setup/teardown and is safe because tests run sequentially.

Results

Metric	Before	After	Improvement
FastAPI test suite	7.50s	1.05s	7.1× faster
Tests passing	210/210	210/210	No regressions

Safety analysis

• dependency_overrides: The mock_authenticated_user fixture sets overrides in setup and calls .clear() in teardown. Since tests run sequentially, no leakage occurs between tests.
• patch scope: The patch("openlibrary.asgi_app.set_context_from_fastapi") now spans the entire session, but the three test files that don't use fastapi_client (test_models.py, test_auth.py, test_list_view_json.py) create their own standalone FastAPI() instances that never reference this function, so they're unaffected.
• Cleanup: client.close() is called at session teardown via the existing try/finally block.

Testing

# FastAPI tests pass
uv run --with-requirements requirements_test.txt \
  pytest openlibrary/tests/fastapi/ -v
# → 210 passed

# Benchmark
uv run --with-requirements requirements_test.txt \
  pytest openlibrary/tests/fastapi/ -q
# → 1.05s (down from 7.50s)

[Copilot]

Pull request overview

Changes the fastapi_client pytest fixture from default function scope to session scope so the FastAPI app (with all routers, middlewares, and static mounts) is built only once per test session, reducing the FastAPI test suite runtime from ~7.5s to ~1.05s.

Changes:

• Add scope="session" to the fastapi_client fixture and update its docstring.

[Sanket17052006]

Tested locally :
On master branch -
docker compose run --rm home pytest openlibrary/tests/fastapi/ -q
210 passed, 3 warnings in 5.90s
On PR branch -
docker compose run --rm home pytest openlibrary/tests/fastapi/ -q
210 passed, 3 warnings in 0.95s

[RayBB]

Thanks for reviewing!