Migrating to async Flask - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

Understanding async route behavior in Flask

What will be the output behavior of this async Flask route when accessed?

from flask import Flask
import asyncio

app = Flask(__name__)

@app.route('/wait')
async def wait():
    await asyncio.sleep(1)
    return 'Done waiting!'

Flask

from flask import Flask
import asyncio

app = Flask(__name__)

@app.route('/wait')
async def wait():
    await asyncio.sleep(1)
    return 'Done waiting!'

A The server waits 1 second asynchronously before responding with 'Done waiting!'

B The server blocks all requests for 1 second before responding with 'Done waiting!'

C The route raises a RuntimeError because async routes are not supported

D The server immediately responds with 'Done waiting!' without delay

💡 Hint

Think about how async functions and await work in Python and Flask's async support.

Async Flask routes allow the server to handle other requests while waiting for awaitable operations like asyncio.sleep. So the server does not block but waits asynchronously.

📝 Syntax

intermediate

Correct async route declaration in Flask

Which of the following is the correct way to declare an async route in Flask 2.0+?

A

@app.route('/data')
async def data():
    return 'Async data'

B

@app.route('/data')
def data():
    await asyncio.sleep(1)
    return 'Async data'

C

@app.route('/data')
async def data():
    return await 'Async data'

D

@app.route('/data')
async def data():
    await asyncio.sleep(1)
    return 'Async data'

💡 Hint

Remember async functions must use await for async calls and return normal values.

Option D correctly defines an async function with await inside and returns a string. Option D misses await, B is not async but uses await (syntax error), D tries to await a string (TypeError).

🔧 Debug

advanced

Identify the error in this async Flask route

What error will this Flask async route raise when accessed?

from flask import Flask

app = Flask(__name__)

@app.route('/error')
async def error():
    return await 'Hello'

Flask

from flask import Flask

app = Flask(__name__)

@app.route('/error')
async def error():
    return await 'Hello'

A TypeError: object str can't be used in 'await' expression

B SyntaxError: 'await' outside async function

C RuntimeError: Async route not supported

D No error, returns 'Hello' correctly

💡 Hint

What can you await in Python? Can you await a string?

You can only await awaitable objects like coroutines or futures. Awaiting a string causes a TypeError.

❓ state_output

advanced

Output of async Flask route with shared state

Consider this Flask app with a shared counter:

from flask import Flask
import asyncio

app = Flask(__name__)
counter = 0

@app.route('/count')
async def count():
    global counter
    temp = counter
    await asyncio.sleep(0.1)
    counter = temp + 1
    return str(counter)

What is the expected output after 3 quick requests to '/count'?

Flask

from flask import Flask
import asyncio

app = Flask(__name__)
counter = 0

@app.route('/count')
async def count():
    global counter
    temp = counter
    await asyncio.sleep(0.1)
    counter = temp + 1
    return str(counter)

A Outputs: 1, 2, 3 (each request increments counter correctly)

B Outputs: 1, 1, 1 (race condition causes same value returned)

C Outputs: 3, 3, 3 (counter increments before requests finish)

D Raises RuntimeError due to global variable access in async route

💡 Hint

Think about what happens when multiple async requests read and write the same variable with a delay.

Because of the await delay, multiple requests read the same counter value before any update, causing race conditions and repeated outputs.

🧠 Conceptual

expert

Best practice for database calls in async Flask routes

When migrating a Flask app to async routes, what is the best approach to handle database calls that are blocking?

A Replace database calls with in-memory dictionaries to avoid blocking

B Keep using the blocking database calls directly inside async routes

C Use an async database driver or run blocking calls in a thread executor

D Remove async from routes and keep all calls blocking

💡 Hint

Async routes should not block the event loop; how to handle blocking operations?

To keep async routes efficient, use async database drivers or run blocking calls in separate threads to avoid blocking the event loop.