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

FastAPI provides TestClient (wrapping httpx) for synchronous tests and AsyncClient for async tests. Dependencies can be overridden for testing to inject mocks instead of real databases or services.

# 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

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