WebSocket endpoint creation in FastAPI - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the WebSocket class from FastAPI.

FastAPI

from fastapi import [1]

Drag options to blanks, or click blank then click option'

AWebSocket

BRequest

CDepends

DHTTPException

Attempts:

3 left

💡 Hint

Common Mistakes

Importing unrelated classes like Request or Depends.

Forgetting to import WebSocket causes errors when defining the endpoint.

The WebSocket class is imported from FastAPI to create WebSocket endpoints.

2fill in blank

medium

Complete the code to define a WebSocket endpoint path '/ws'.

FastAPI

@app.websocket("[1]")
def websocket_endpoint(websocket: WebSocket):
    await websocket.accept()

Drag options to blanks, or click blank then click option'

A/socket

B/ws

C/websocket

D/chat

Attempts:

3 left

💡 Hint

Common Mistakes

Using HTTP route decorators like @app.get instead of @app.websocket.

Missing the leading slash in the path string.

The WebSocket endpoint is defined at path /ws using the @app.websocket decorator.

3fill in blank

hard

Fix the error in the code to accept the WebSocket connection properly.

FastAPI

async def websocket_endpoint(websocket: WebSocket):
    await websocket.[1]()

Drag options to blanks, or click blank then click option'

Aconnect

Bstart

Caccept

Dopen

Attempts:

3 left

💡 Hint

Common Mistakes

Using non-existent methods like connect() or open() causes runtime errors.

Forgetting to accept the connection leads to client connection failures.

The method accept() is used to accept an incoming WebSocket connection.

4fill in blank

hard

Fill both blanks to receive a text message and send it back to the client.

FastAPI

data = await websocket.[1]()
await websocket.[2](data)

Drag options to blanks, or click blank then click option'

Areceive_text

Bsend_text

Creceive_bytes

Dsend_bytes

Attempts:

3 left

💡 Hint

Common Mistakes

Using receive_bytes() when expecting text causes type errors.

Sending bytes when client expects text causes message format errors.

Use receive_text() to get a text message and send_text() to send it back.

5fill in blank

hard

Fill all three blanks to create a complete WebSocket echo endpoint that accepts, receives, sends, and closes the connection.

FastAPI

from fastapi import FastAPI, WebSocket

app = FastAPI()

@app.websocket("/echo")
async def echo_endpoint(websocket: WebSocket):
    await websocket.[1]()
    data = await websocket.[2]()
    await websocket.[3](data)
    await websocket.close()

Drag options to blanks, or click blank then click option'

Aaccept

Breceive_text

Csend_text

Dconnect

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'connect' instead of 'accept' causes errors.

Mixing receive_text with send_bytes or vice versa causes message format issues.

This code accepts the WebSocket connection, receives a text message, sends it back, and then closes the connection.

Practice

1. What decorator is used in FastAPI to create a WebSocket endpoint?

easy

A. @app.websocket

B. @app.get

C. @app.post

D. @app.route

Solution

1. Step 1: Understand FastAPI endpoint decorators

   FastAPI uses different decorators for different endpoint types: @app.get for GET HTTP, @app.post for POST HTTP, and @app.websocket for WebSocket connections.

2. Step 2: Identify the correct decorator for WebSocket

   Since WebSocket is a different protocol from HTTP, FastAPI uses @app.websocket to create WebSocket endpoints.

3. Final Answer:

   @app.websocket -> Option A

4. Quick Check:

   WebSocket endpoint decorator = @app.websocket [OK]

Hint: WebSocket endpoints use @app.websocket decorator [OK]

Common Mistakes:

• Using @app.get or @app.post instead of @app.websocket
• Confusing HTTP routes with WebSocket routes
• Trying to use @app.route which is not in FastAPI

2. Which of the following is the correct syntax to accept a WebSocket connection in FastAPI?

easy

A. await websocket.connect()

B. websocket.accept()

C. await websocket.accept()

D. websocket.connect()

Solution

1. Step 1: Recognize asynchronous WebSocket methods

   FastAPI WebSocket methods are async, so you must use await to call them.

2. Step 2: Identify the method to accept a connection

   The method to accept a WebSocket connection is accept(), which must be awaited as await websocket.accept().

3. Final Answer:

   await websocket.accept() -> Option C

4. Quick Check:

   Accept WebSocket connection = await websocket.accept() [OK]

Hint: Use await with websocket.accept() to accept connection [OK]

Common Mistakes:

• Forgetting to use await with async methods
• Using websocket.connect() which does not exist
• Calling accept() without await causing runtime errors

3. Given this FastAPI WebSocket endpoint code, what will it send back to the client when the client sends 'hello'?

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 text was: {data}")

medium

A. Message text was: hello

B. hello

C. Message text was: None

D. Error: receive_text() not awaited

Solution

1. Step 1: Trace the WebSocket message flow

   The server accepts the connection, then waits to receive text from the client. It stores the received text in 'data'.

2. Step 2: Understand the response sent back

   The server sends back a message with the text 'Message text was: ' plus the received data. If client sends 'hello', the response is 'Message text was: hello'.

3. Final Answer:

   Message text was: hello -> Option A

4. Quick Check:

   Sent message = 'Message text was: hello' [OK]

Hint: Server echoes with prefix 'Message text was:' [OK]

Common Mistakes:

• Confusing received message with sent message
• Missing await causing runtime errors
• Assuming server sends back raw client message without prefix

4. Identify the error in this FastAPI WebSocket endpoint code:

from fastapi import FastAPI, WebSocket
app = FastAPI()

@app.websocket("/ws")
async def websocket_endpoint(websocket: WebSocket):
    websocket.accept()
    data = await websocket.receive_text()
    await websocket.send_text(data)

medium

A. Missing await before websocket.receive_text()

B. Missing await before websocket.accept()

C. Missing await before websocket.send_text()

D. Incorrect WebSocket path

Solution

1. Step 1: Check async method calls

   In FastAPI, websocket.accept() is an async method and must be awaited.

2. Step 2: Identify missing await

   The code calls websocket.accept() without await, which will cause a runtime error.

3. Final Answer:

   Missing await before websocket.accept() -> Option B

4. Quick Check:

   Async methods require await = missing await [OK]

Hint: Always await async WebSocket methods like accept() [OK]

Common Mistakes:

• Calling async methods without await
• Confusing which methods are async
• Assuming accept() is synchronous

5. You want to create a FastAPI WebSocket endpoint that continuously receives messages and sends back each message reversed. Which code snippet correctly implements this behavior?

hard

A. async def websocket_endpoint(websocket: WebSocket):
websocket.accept()
while True:
data = websocket.receive_text()
websocket.send_text(data[::-1])

B. async def websocket_endpoint(websocket: WebSocket):
await websocket.accept()
data = await websocket.receive_text()
await websocket.send_text(data[::-1])

C. async def websocket_endpoint(websocket: WebSocket):
await websocket.accept()
for data in websocket.receive_text():
await websocket.send_text(data[::-1])

D. async def websocket_endpoint(websocket: WebSocket):
await websocket.accept()
while True:
data = await websocket.receive_text()
await websocket.send_text(data[::-1])

Solution

1. Step 1: Understand continuous message handling

   To handle multiple messages, the server must loop indefinitely, receiving and sending messages repeatedly.

2. Step 2: Check async calls and loop structure

   async def websocket_endpoint(websocket: WebSocket): await websocket.accept() while True: data = await websocket.receive_text() await websocket.send_text(data[::-1]) uses an async while True loop, awaits receive_text(), and sends back reversed text with await send_text(), correctly handling async and continuous flow.

3. Final Answer:

   async def websocket_endpoint(websocket: WebSocket): await websocket.accept() while True: data = await websocket.receive_text() await websocket.send_text(data[::-1]) -> Option D

4. Quick Check:

   Continuous async loop with await calls = async def websocket_endpoint(websocket: WebSocket): await websocket.accept() while True: data = await websocket.receive_text() await websocket.send_text(data[::-1]) [OK]

Hint: Use while True with await receive and send for continuous WebSocket [OK]

Common Mistakes:

• Missing loop for continuous messages
• Not awaiting async methods
• Using for loop incorrectly on async receive_text()
• Calling async methods without await