Testing a FastAPI endpoint with dependency_overrides

# ======================================================================
# Q: How do you test a FastAPI endpoint without a real database and without real
# auth?
#
# app.dependency_overrides replaces any Depends, so I do not need monkeypatch.
# For the database I prefer real PostgreSQL (Docker or testcontainers) with a
# transaction that rolls back at the end of each test, because SQLite does not
# have the same constraints, JSONB and locking. In Django the equivalent is
# pytest-django with @pytest.mark.django_db.
# ======================================================================

from fastapi.testclient import TestClient
from app.main import app
from app.deps import get_session, current_user, get_erp

@pytest.fixture
def client(db_session):                       # db_session: a transaction with rollback
    fake_erp = FakeErpClient()
    app.dependency_overrides[get_session] = lambda: db_session
    app.dependency_overrides[current_user] = lambda: User(id=1, tenant_id=7,
                                                          perms={'orders:write'})
    app.dependency_overrides[get_erp] = lambda: fake_erp
    with TestClient(app) as c:
        c.fake_erp = fake_erp
        yield c
    app.dependency_overrides.clear()

def test_create_order_pushes_to_erp(client):
    r = client.post('/v1/orders', json={'lines': [{'sku': 'A1', 'qty': 2}]})
    assert r.status_code == 201
    assert client.fake_erp.sent[0][0] == r.json()['id']

def test_validation_error(client):
    r = client.post('/v1/orders', json={'lines': [{'sku': 'A1', 'qty': 0}]})
    assert r.status_code == 422