Dependencies in Python

Depends injects shared setup, and the framework caches it per request.

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

app = FastAPI()


def get_db():
    db = {"rows": ["python", "rust"]}
    try:
        yield db
    finally:
        db["closed"] = True


def pagination(limit: int = 10, offset: int = 0):
    return {"limit": min(limit, 50), "offset": offset}


@app.get("/langs")
def langs(db=Depends(get_db), page=Depends(pagination)):
    rows = db["rows"][page["offset"]:page["offset"] + page["limit"]]
    return {"rows": rows, "page": page}


client = TestClient(app)
print(client.get("/langs", params={"limit": 1}).json())

app.dependency_overrides[get_db] = lambda: {"rows": ["fake"]}
print(client.get("/langs").json()["rows"])

How it works

1. A dependency is just a callable with its own parameters.
2. It can yield, which gives it teardown.
3. Overriding one is how tests replace a database.

Keywords and builtins used here

• def
• finally
• get_db
• int
• lambda
• langs
• min
• pagination
• print
• return
• try
• yield

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Step 1 of 3 in Dependencies & async, step 10 of 19 in Web services & data access.

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