WebSocket endpoint creation in FastAPI - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when a client sends 'hello' to this WebSocket endpoint?

Consider this FastAPI WebSocket endpoint code. What will the client receive after sending the message 'hello'?

FastAPI

from fastapi import FastAPI, WebSocket

app = FastAPI()

@app.websocket('/ws')
async def websocket_endpoint(websocket: WebSocket):
    await websocket.accept()
    data = await websocket.receive_text()
    await websocket.send_text(f'Message received: {data}')
    await websocket.close()

A RuntimeError due to missing await

B "hello"

C "Message received: hello"

D Connection closed without response

💡 Hint

Look at what the server sends back after receiving the message.

The server accepts the connection, receives the text 'hello', then sends back 'Message received: hello' before closing.

📝 Syntax

intermediate

Which option correctly defines a FastAPI WebSocket endpoint?

Select the option that correctly defines a WebSocket endpoint in FastAPI without syntax errors.

A

from fastapi import FastAPI, WebSocket
app = FastAPI()
@app.websocket('/chat')
async def chat(websocket: WebSocket):
    await websocket.accept()
    await websocket.send_text('Welcome!')

B

from fastapi import FastAPI, WebSocket
app = FastAPI()
@app.websocket('/chat')
def chat(websocket: WebSocket):
    websocket.accept()
    websocket.send_text('Welcome!')

C

from fastapi import FastAPI, WebSocket
app = FastAPI()
@app.websocket('/chat')
async def chat():
    await websocket.accept()
    await websocket.send_text('Welcome!')

D

from fastapi import FastAPI, WebSocket
app = FastAPI()
@app.websocket('/chat')
async def chat(websocket):
    await websocket.accept()
    await websocket.send_text('Welcome!')

💡 Hint

Check for async keyword and parameter type.

Option A correctly uses async def with a typed websocket parameter and awaits calls. Others miss async, parameter type, or websocket parameter.

❓ state_output

advanced

What is the value of 'count' after 3 messages are received in this WebSocket endpoint?

Given this WebSocket endpoint, what will be the value of 'count' after the client sends 3 messages?

FastAPI

from fastapi import FastAPI, WebSocket

app = FastAPI()

count = 0

@app.websocket('/counter')
async def counter(websocket: WebSocket):
    global count
    await websocket.accept()
    for _ in range(3):
        await websocket.receive_text()
        count += 1
    await websocket.send_text(f'Count is {count}')
    await websocket.close()

A Raises UnboundLocalError

B 0

C 1

D 3

💡 Hint

Look at how 'count' is updated inside the loop.

The global 'count' is incremented by 1 for each of the 3 messages, so final value is 3.

🔧 Debug

advanced

Which option correctly describes the behavior when connecting to this WebSocket endpoint?

Identify which option correctly describes the behavior when a client connects to this WebSocket endpoint.

FastAPI

from fastapi import FastAPI, WebSocket

app = FastAPI()

@app.websocket('/echo')
async def echo(websocket: WebSocket):
    await websocket.accept()
    while True:
        data = await websocket.receive_text()
        await websocket.send_text(data)

A Client connects but server never accepts connection.

B Client sends 'test' and receives 'test' back repeatedly.

C Client connects and server closes connection immediately.

D Client connects and server raises RuntimeError due to missing await.

💡 Hint

Check if the server accepts connection and awaits properly.

Option B describes the correct behavior of this code: server accepts, echoes messages in a loop.

🧠 Conceptual

expert

What happens if you omit 'await websocket.accept()' in a FastAPI WebSocket endpoint?

In FastAPI, what is the effect of not calling 'await websocket.accept()' inside a WebSocket endpoint function?

A The client connection is rejected and the handshake fails.

B The server accepts the connection automatically without explicit accept call.

C The server sends a default welcome message to the client.

D The server raises a SyntaxError at runtime.

💡 Hint

Think about the WebSocket handshake process.

Without calling 'await websocket.accept()', the WebSocket handshake is not completed, so the client connection is rejected.