WebSocket endpoint creation in FastAPI

Introduction

WebSocket endpoints let your app talk to users instantly without waiting. This is great for live chats or real-time updates.

You want to build a live chat app where messages appear instantly.

You need to show real-time notifications or alerts to users.

You want to update a dashboard with live data without refreshing the page.

You are making a multiplayer game that needs fast communication.

You want to stream data continuously, like stock prices or sensor info.

Syntax

FastAPI

from fastapi import FastAPI, WebSocket

app = FastAPI()

@app.websocket("/ws")
async def websocket_endpoint(websocket: WebSocket):
    await websocket.accept()
    while True:
        data = await websocket.receive_text()
        await websocket.send_text(f"Message text was: {data}")

Use @app.websocket("/path") to create a WebSocket endpoint.

Call await websocket.accept() to accept the connection before communication.

Examples

A simple WebSocket that accepts one message and replies back.

FastAPI

from fastapi import FastAPI, WebSocket

app = FastAPI()

@app.websocket("/chat")
async def chat(websocket: WebSocket):
    await websocket.accept()
    data = await websocket.receive_text()
    await websocket.send_text(f"You said: {data}")

This endpoint echoes back every message it receives in a loop.

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)

Sample Program

This program creates a WebSocket endpoint at /ws. It accepts connections, waits for text messages, and sends back a reply showing what was received.

FastAPI

from fastapi import FastAPI, WebSocket

app = FastAPI()

@app.websocket("/ws")
async def websocket_endpoint(websocket: WebSocket):
    await websocket.accept()
    while True:
        data = await websocket.receive_text()
        await websocket.send_text(f"Message text was: {data}")

Important Notes

Always call await websocket.accept() to start the connection.

Use await websocket.receive_text() to get messages from the client.

Use await websocket.send_text() to send messages back.

Summary

WebSocket endpoints let your app communicate instantly with users.

Use @app.websocket decorator to create them in FastAPI.

Remember to accept connections and handle messages asynchronously.

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