Testing With Dependency Overrides

app.dependency_overrides

FastAPI tests use TestClient (or httpx's AsyncClient for async tests) with pytest. Because resources come from dependencies, tests replace them through app.dependency_overrides[real] = fake: a fake database, a stub payment client, a fixed current user. This keeps tests fast and isolated without patching internals. Clear overrides after each test, and add a few integration tests against a real database.

Overriding a database dependency, run

I ran this with Python 3.12, FastAPI 0.142.2, Pydantic 2.13 and Starlette 1.7, calling the app through FastAPI's TestClient (no server needed). The real get_db would fail, but the override supplies an in-memory dict with two products, so the endpoint returns a count of 2.

from typing import Annotated
from fastapi import Depends, FastAPI
from fastapi.testclient import TestClient

def get_db():
    raise RuntimeError("would connect to the real database")

app = FastAPI()

@app.get("/count")
def count(db: Annotated[dict, Depends(get_db)]):
    return {"products": len(db["products"])}

app.dependency_overrides[get_db] = lambda: {"products": ["pen", "ink"]}   # test double
client = TestClient(app)
print(client.get("/count").json())

Output:

{'products': 2}

Reset overrides between tests

Use a pytest fixture that sets overrides and clears app.dependency_overrides afterwards.

Quick check: How do you replace a database dependency in a FastAPI test?

• Disable Pydantic
• Edit the source file during the test
• Set app.dependency_overrides[get_db] to a fake
• Restart uvicorn