WebSocket endpoint creation in FastAPI - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a WebSocket endpoint in FastAPI?

A WebSocket endpoint in FastAPI is a special route that allows real-time, two-way communication between the client and server using the WebSocket protocol.

beginner

How do you accept a WebSocket connection in FastAPI?

You accept a WebSocket connection by calling the await websocket.accept() method inside the WebSocket endpoint function.

beginner

Which FastAPI class is used to handle WebSocket connections?

The <code>WebSocket</code> class from <code>fastapi</code> is used to handle WebSocket connections, allowing sending and receiving messages.

beginner

What method do you use to receive a message from a WebSocket client in FastAPI?

Use await websocket.receive_text() to receive a text message from the client.

beginner

How do you send a message back to the WebSocket client in FastAPI?

Use await websocket.send_text(message) to send a text message back to the client.

Which decorator is used to create a WebSocket endpoint in FastAPI?

A @app.websocket

B @app.get

C @app.post

D @app.route

The @app.websocket decorator defines a WebSocket endpoint in FastAPI.

What must you do first inside a WebSocket endpoint function to start communication?

A Call websocket.accept()

B Call websocket.close()

C Call websocket.send_text()

D Call websocket.receive_text()

You must call await websocket.accept() to accept the connection before sending or receiving messages.

How do you receive a text message from the client in FastAPI WebSocket?

A await websocket.accept()

B await websocket.send_text()

C await websocket.close()

D await websocket.receive_text()

Use await websocket.receive_text() to get a text message from the client.

Which protocol does FastAPI use for WebSocket communication?

A FTP

B HTTP/1.1

C WebSocket protocol

D SMTP

FastAPI uses the WebSocket protocol for real-time two-way communication.

What happens if you don't call websocket.accept() in a FastAPI WebSocket endpoint?

A The server crashes

B The connection is not established

C Messages can still be sent and received

D The connection closes immediately

Without calling websocket.accept(), the WebSocket connection is not established and communication cannot start.

Explain step-by-step how to create a WebSocket endpoint in FastAPI that echoes messages back to the client.

Think about accepting connection, receiving, and sending messages.

Describe the role of the WebSocket class in FastAPI and how it helps in real-time communication.

Focus on what the WebSocket object lets you do inside the endpoint.

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