Async test patterns in FastAPI - Mini Project: Build & Apply

Async Test Patterns with FastAPI

📖 Scenario: You are building a simple FastAPI web service that returns user data asynchronously. You want to write tests to check that your async endpoint works correctly.

🎯 Goal: Create a FastAPI app with an async endpoint, then write an async test function using AsyncClient from httpx to call the endpoint and verify the response.

📋 What You'll Learn

Create a FastAPI app with an async GET endpoint at /user that returns a JSON with {"name": "Alice", "age": 30}

Create a test function using pytest and httpx.AsyncClient to call the /user endpoint asynchronously

Use async def for the test function and await the client call

Assert the response status code is 200 and the JSON matches the expected data

💡 Why This Matters

🌍 Real World

Async testing is essential for modern web APIs that handle many requests concurrently. FastAPI uses async functions to improve performance, so testing async endpoints correctly ensures your API works reliably.

💼 Career

Many backend developer roles require writing tests for async web services. Knowing how to write async tests with FastAPI and httpx is a valuable skill for ensuring code quality and catching bugs early.

1

Create FastAPI app with async endpoint

Create a FastAPI app called app and add an async GET endpoint at /user that returns {"name": "Alice", "age": 30} as JSON.

FastAPI

from fastapi import FastAPI

# Your code here

Need a hint?

Use FastAPI() to create the app. Define an async function with @app.get("/user") decorator that returns the dictionary.

2

Import AsyncClient and pytest

Import AsyncClient from httpx and pytest to prepare for writing async tests.

FastAPI

from fastapi import FastAPI
import pytest
from httpx import AsyncClient

app = FastAPI()

@app.get("/user")
async def get_user():
    return {"name": "Alice", "age": 30}

# Your code here

Need a hint?

Use import pytest and from httpx import AsyncClient at the top of your file.

3

Write async test function using AsyncClient

Write an async test function called test_get_user that uses AsyncClient with app as the app argument. Inside, await a GET request to /user and store the response in response.

FastAPI

from fastapi import FastAPI
import pytest
from httpx import AsyncClient

app = FastAPI()

@app.get("/user")
async def get_user():
    return {"name": "Alice", "age": 30}

@pytest.mark.asyncio
async def test_get_user():
    async with AsyncClient(app=app, base_url="http://test") as client:
        # Your code here

Need a hint?

Use async def for the test function. Use async with AsyncClient(app=app, base_url="http://test") as client to create the client. Then await client.get("/user").

4

Assert response status and JSON content

Inside test_get_user, add assertions to check that response.status_code is 200 and response.json() equals {"name": "Alice", "age": 30}.

FastAPI

from fastapi import FastAPI
import pytest
from httpx import AsyncClient

app = FastAPI()

@app.get("/user")
async def get_user():
    return {"name": "Alice", "age": 30}

@pytest.mark.asyncio
async def test_get_user():
    async with AsyncClient(app=app, base_url="http://test") as client:
        response = await client.get("/user")
        # Your code here

Need a hint?

Use assert response.status_code == 200 and assert response.json() == {"name": "Alice", "age": 30} inside the test function.