FastAPI background tasks + dependency overrides: working versions (tested 2026-10-15)

fastapi==0.143.1
httpx==0.28.1
pydantic==2.14.1
pytest==9.1.1

from fastapi import BackgroundTasks, Depends, FastAPI
from pydantic import BaseModel

class Mailer:
    def send(self, to: str, subject: str) -> None:  # real SMTP/API call goes here
        raise RuntimeError("configure a real mailer")

def get_mailer() -> Mailer:
    return Mailer()

class Signup(BaseModel):
    email: str

app = FastAPI()

@app.post("/signup", status_code=202)
def signup(data: Signup, tasks: BackgroundTasks, mailer: Mailer = Depends(get_mailer)):
    tasks.add_task(mailer.send, data.email, "Welcome!")
    return {"queued": True}

from fastapi.testclient import TestClient
from app.main import Mailer, app, get_mailer

class FakeMailer(Mailer):
    def __init__(self):
        self.sent = []
    def send(self, to, subject):
        self.sent.append((to, subject))

def test_background_task_runs_after_response():
    fake = FakeMailer()
    app.dependency_overrides[get_mailer] = lambda: fake
    try:
        r = TestClient(app).post("/signup", json={"email": "[email protected]"})
        assert r.status_code == 202
        assert fake.sent == [("[email protected]", "Welcome!")]
    finally:
        app.dependency_overrides.clear()

[pytest]
pythonpath = .