Async test patterns in FastAPI - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this async FastAPI test?

Consider this FastAPI async test using httpx.AsyncClient. What will be the printed output when running this test?

FastAPI

from fastapi import FastAPI
from fastapi.testclient import TestClient
import pytest
import httpx

app = FastAPI()

@app.get("/hello")
async def hello():
    return {"message": "Hello, async!"}

@pytest.mark.asyncio
async def test_hello():
    async with httpx.AsyncClient(app=app, base_url="http://test") as client:
        response = await client.get("/hello")
        print(response.json())
        assert response.status_code == 200

A SyntaxError due to missing await

B {"message": "Hello, async!"}

C TypeError because AsyncClient is not awaited

D {"message": "Hello, sync!"}

💡 Hint

Remember that httpx.AsyncClient supports async context and requests must be awaited.

The test uses httpx.AsyncClient correctly with async context and awaits the GET request. The endpoint returns the JSON with message 'Hello, async!'. So the printed output is the JSON dictionary.

📝 Syntax

intermediate

Which option causes a syntax error in async FastAPI test?

Identify which test code snippet will cause a syntax error when testing an async FastAPI endpoint.

FastAPI

from fastapi import FastAPI
import pytest
import httpx

app = FastAPI()

@app.get("/data")
async def get_data():
    return {"data": 123}

@pytest.mark.asyncio
async def test_data():
    async with httpx.AsyncClient(app=app, base_url="http://test") as client:
        response = await client.get("/data")
        assert response.status_code == 200

A

async def test_data():
    async with httpx.AsyncClient(app=app, base_url="http://test") as client:
        response = await client.get("/data")
        assert response.status_code == 200

B

async def test_data():
    async with httpx.AsyncClient(app=app) as client:
        response = await client.get("/data")
        assert response.status_code == 200

C

async def test_data():
    async with httpx.AsyncClient(app=app) as client
        response = await client.get("/data")
        assert response.status_code == 200

D

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💡 Hint

Check for missing colons in async context managers.

Option C is missing a colon after the async with statement, causing a syntax error. The other options have correct syntax.

🔧 Debug

advanced

Why does this async test raise RuntimeError?

This async FastAPI test raises RuntimeError: This event loop is already running. What is the cause?

FastAPI

import asyncio
import pytest
from fastapi import FastAPI
import httpx

app = FastAPI()

@pytest.mark.asyncio
async def test_loop():
    loop = asyncio.get_event_loop()
    async with httpx.AsyncClient(app=app, base_url="http://test") as client:
        response = loop.run_until_complete(client.get("/"))
        assert response.status_code == 200

A FastAPI app is not started causing RuntimeError

B Missing await keyword before client.get causes RuntimeError

C httpx.AsyncClient requires sync client in async tests causing RuntimeError

D Using run_until_complete inside an already running async test causes RuntimeError

💡 Hint

Think about event loops and how async tests run.

The test function is async and runs inside an event loop. Calling run_until_complete inside it tries to start a new loop on top of the running one, causing RuntimeError.

❓ state_output

advanced

What is the value of counter after running this async test?

Given this FastAPI app with a shared counter, what is the final value of counter after the test runs?

FastAPI

from fastapi import FastAPI
import pytest
import httpx

app = FastAPI()
counter = 0

@app.post("/increment")
async def increment():
    global counter
    counter += 1
    return {"count": counter}

@pytest.mark.asyncio
async def test_increment():
    async with httpx.AsyncClient(app=app, base_url="http://test") as client:
        await client.post("/increment")
        await client.post("/increment")
        await client.post("/increment")

A 3

B 1

C 0

D Raises UnboundLocalError

💡 Hint

Check how the global variable is modified in the async endpoint.

The global counter is incremented three times by the three POST requests, so its final value is 3.

🧠 Conceptual

expert

Which option correctly explains async test isolation in FastAPI?

In async FastAPI tests, why is it important to use fixtures or setup/teardown to isolate test state?

A Because async tests run concurrently, shared state can cause flaky tests without isolation

B Because async tests run sequentially, shared state does not affect test results

C Because FastAPI automatically resets global variables between tests, isolation is not needed

D Because httpx.AsyncClient creates a new event loop for each test, state is isolated by default

💡 Hint

Think about how async tests might run in parallel and affect shared data.

Async tests can run concurrently, so if they share mutable global state without isolation, tests can interfere and produce unreliable results.