How do you write tests for a FastAPI application using pytest and TestClient?

# app/main.py
from fastapi import FastAPI, Depends
from pydantic import BaseModel

app = FastAPI()

def get_db():
    return {"connection": "real_db"}

class Item(BaseModel):
    name: str
    price: float

@app.get("/")
def root():
    return {"message": "Hello"}

@app.post("/items", status_code=201)
def create_item(item: Item, db=Depends(get_db)):
    return {"created": item.name, "db": db["connection"]}

# tests/test_main.py
import pytest
from fastapi.testclient import TestClient
from app.main import app, get_db

# Override the real DB dependency with a mock
def override_get_db():
    return {"connection": "test_db"}

app.dependency_overrides[get_db] = override_get_db

client = TestClient(app)

def test_root():
    response = client.get("/")
    assert response.status_code == 200
    assert response.json() == {"message": "Hello"}

def test_create_item():
    response = client.post(
        "/items",
        json={"name": "Widget", "price": 9.99},
    )
    assert response.status_code == 201
    data = response.json()
    assert data["created"] == "Widget"
    assert data["db"] == "test_db"  # mock dependency used

def test_create_item_invalid():
    response = client.post("/items", json={"name": "Widget"})  # missing price
    assert response.status_code == 422  # validation error

# Async test with httpx.AsyncClient
import pytest
from httpx import AsyncClient, ASGITransport

@pytest.mark.asyncio
async def test_root_async():
    async with AsyncClient(
        transport=ASGITransport(app=app), base_url="http://test"
    ) as ac:
        response = await ac.get("/")
    assert response.status_code == 200