Async path operations in FastAPI - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this async FastAPI endpoint?

Consider this FastAPI async endpoint. What will the client receive when calling /hello?

FastAPI

from fastapi import FastAPI
import asyncio

app = FastAPI()

@app.get('/hello')
async def say_hello():
    await asyncio.sleep(0.1)
    return {'message': 'Hello, async world!'}

A {"message": "Hello, async world!"}

B SyntaxError due to missing await

C {"message": "Hello, world!"}

D RuntimeError because asyncio.sleep is blocking

💡 Hint

Remember async functions can use await to pause without blocking.

The async endpoint uses await asyncio.sleep(0.1) to pause briefly without blocking. It then returns the JSON with the message 'Hello, async world!'.

📝 Syntax

intermediate

Which option correctly defines an async FastAPI path operation?

Select the option that correctly defines an async GET endpoint in FastAPI that returns a JSON with key 'status' and value 'ok'.

A

from fastapi import FastAPI
app = FastAPI()

@app.get('/status')
async def status():
    return {'status': 'ok'}

B

from fastapi import FastAPI
app = FastAPI()

@app.get('/status')
def status():
    await return {'status': 'ok'}

C

from fastapi import FastAPI
app = FastAPI()

@app.get('/status')
async def status():
    return await {'status': 'ok'}

D

from fastapi import FastAPI
app = FastAPI()

@app.get('/status')
async def status():
    await {'status': 'ok'}

💡 Hint

Only async functions can use await, and await must be used with awaitable objects.

Option A defines an async function that returns a dictionary directly. Options A, B, and D misuse await or have syntax errors.

❓ state_output

advanced

What is the output when calling this async endpoint twice quickly?

Given this FastAPI async endpoint, what will be the output of two quick calls to /counter?

FastAPI

from fastapi import FastAPI
import asyncio

app = FastAPI()
counter = 0

@app.get('/counter')
async def get_counter():
    global counter
    await asyncio.sleep(0.05)
    counter += 1
    return {'count': counter}

A [{"count": 1}, {"count": 1}]

B [{"count": 1}, {"count": 2}]

C [{"count": 2}, {"count": 3}]

D RuntimeError due to race condition

💡 Hint

Think about how async functions run and how global variables update.

Each call awaits 0.05 seconds, then increments the global counter. The first call returns 1, the second returns 2.

🔧 Debug

advanced

Which option causes a runtime error in this async FastAPI endpoint?

Identify which option will cause a runtime error when the endpoint /data is called.

FastAPI

from fastapi import FastAPI
app = FastAPI()

@app.get('/data')
async def get_data():
    data = {'a': 1, 'b': 2}
    result = await data.get('a')
    return {'result': result}

A SyntaxError due to missing colon

B Returns {'result': 1}

C TypeError: object int can't be used in 'await' expression

D KeyError because 'a' is missing

💡 Hint

Remember await only works with awaitable objects like coroutines.

The code tries to await an integer (data.get('a') returns 1), which is not awaitable, causing a TypeError.

🧠 Conceptual

expert

Why should FastAPI path operations be async when performing I/O?

Select the best explanation why defining FastAPI path operations as async functions is important when they perform I/O operations like database queries or HTTP calls.

A Async path operations automatically cache I/O results to speed up responses.

B Async path operations prevent any exceptions from occurring during I/O.

C Async path operations convert blocking I/O calls into CPU-bound tasks to speed up processing.

D Async path operations allow FastAPI to handle other requests while waiting for I/O, improving concurrency and throughput.

💡 Hint

Think about what happens when a server waits for slow tasks.

Async functions let the server pause on slow I/O without blocking other requests, improving efficiency and user experience.