# Testing With Dependency Overrides — Django / FastAPI

Source: https://www.skillbyai.com/en/django-fastapi/p-testing

> Swap real resources for test doubles.

## 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.

```python
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.

**Quiz:** How do you replace a database dependency in a FastAPI test?

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

*Answer:* Set app.dependency_overrides[get_db] to a fake. Dependencies make testing easy.