Sending and receiving messages in FastAPI - Mini Project: Build & Apply

Sending and receiving messages

📖 Scenario: You are building a simple messaging API where users can send and receive messages. This API will allow clients to post new messages and get all messages sent so far.

🎯 Goal: Create a FastAPI app that stores messages in memory, allows adding new messages via POST, and retrieving all messages via GET.

📋 What You'll Learn

Create a list called messages to store message strings

Create a FastAPI app instance called app

Add a POST endpoint /send that accepts a JSON body with a content field and appends it to messages

Add a GET endpoint /receive that returns all stored messages as JSON

💡 Why This Matters

🌍 Real World

Messaging APIs are common in chat apps, notifications, and communication platforms.

💼 Career

Understanding how to build simple REST APIs with FastAPI is useful for backend development roles.

1

Data setup: Create the messages list

Create a list called messages that is initially empty to store message strings.

FastAPI

# Create an empty list called messages
# Your code here

Hint

Use messages = [] to create an empty list.

2

Configuration: Create the FastAPI app instance

Import FastAPI from fastapi and create an app instance called app.

FastAPI

messages = []
# Import FastAPI and create app instance
# Your code here

Hint

Use from fastapi import FastAPI and then app = FastAPI().

3

Core logic: Add POST endpoint to send messages

Add a POST endpoint /send to app that accepts a JSON body with a content field and appends it to the messages list. Use a function named send_message with a parameter message of type dict.

FastAPI

from fastapi import FastAPI

messages = []
app = FastAPI()

# Add POST endpoint /send to append message content
# Your code here

Hint

Use @app.post("/send") decorator and an async function that appends message["content"] to messages.

4

Completion: Add GET endpoint to receive all messages

Add a GET endpoint /receive to app that returns the entire messages list as JSON. Use a function named receive_messages.

FastAPI

from fastapi import FastAPI

messages = []
app = FastAPI()

@app.post("/send")
async def send_message(message: dict):
    messages.append(message["content"])
    return {"status": "Message received"}

# Add GET endpoint /receive to return all messages
# Your code here

Hint

Use @app.get("/receive") decorator and return {"messages": messages}.

Practice

1. What is the main purpose of using Pydantic models in FastAPI when sending and receiving messages?

easy

A. To create HTML templates for message display

B. To manage user authentication tokens

C. To handle database connections automatically

D. To clearly define the structure and data types of messages

Solution

1. Step 1: Understand Pydantic's role in FastAPI

   Pydantic models define how data should look, including types and required fields.

2. Step 2: Connect Pydantic to message handling

   Using Pydantic ensures messages sent or received follow a clear, expected format.

3. Final Answer:

   To clearly define the structure and data types of messages -> Option D

4. Quick Check:

   Pydantic models = message structure [OK]

Hint: Pydantic models describe message shape and types [OK]

Common Mistakes:

• Confusing Pydantic with HTML templating
• Thinking Pydantic manages databases
• Assuming Pydantic handles authentication

2. Which of the following is the correct way to declare a POST endpoint in FastAPI that receives a message model named Message?

easy

A. @app.post('/send')\ndef send_message():\n return Message

B. @app.post('/send')\nasync def send_message(msg: Message):\n return msg

C. @app.get('/send')\nasync def send_message(msg: Message):\n return msg

D. @app.put('/send')\nasync def send_message(msg: Message):\n return msg

Solution

1. Step 1: Identify the HTTP method for sending messages

   POST is used to send data to the server, so @app.post is correct.

2. Step 2: Check function signature for receiving a Pydantic model

   Function must accept a parameter typed as Message to receive the message body.

3. Final Answer:

   @app.post('/send')\nasync def send_message(msg: Message):\n return msg -> Option B

4. Quick Check:

   POST + typed param = correct endpoint [OK]

Hint: Use @app.post and typed parameter for message body [OK]

Common Mistakes:

• Using GET instead of POST for sending data
• Missing parameter type annotation
• Using wrong HTTP method like PUT

3. Given this FastAPI endpoint code, what will be the response when sending a POST request with JSON {"text": "Hello"}?

from fastapi import FastAPI
from pydantic import BaseModel

app = FastAPI()

class Message(BaseModel):
    text: str

@app.post('/echo')
async def echo_message(msg: Message):
    return {"message": msg.text.upper()}

medium

A. {"message": "HELLO"}

B. {"message": "Hello"}

C. {"text": "HELLO"}

D. 500 Internal Server Error

Solution

1. Step 1: Understand the input and model

   The input JSON has key "text" with value "Hello", matching Message model.

2. Step 2: Analyze the endpoint return

   The function returns a dict with key "message" and value as msg.text.upper(), which converts "Hello" to "HELLO".

3. Final Answer:

   {"message": "HELLO"} -> Option A

4. Quick Check:

   Uppercase text returned in 'message' key [OK]

Hint: Look for .upper() call on input text [OK]

Common Mistakes:

• Returning original text without uppercasing
• Returning wrong key name in response
• Expecting an error instead of valid response

4. What is wrong with this FastAPI endpoint code for receiving a message?

from fastapi import FastAPI
from pydantic import BaseModel

app = FastAPI()

class Message(BaseModel):
    content: str

@app.post('/receive')
def receive_message(msg: Message):
    return {"received": msg.content}

medium

A. The endpoint function is missing async keyword

B. The parameter msg should be annotated with Request

C. The endpoint must use @app.get instead of @app.post

D. The function must return a Pydantic model instance

Solution

1. Step 1: Check function declaration for FastAPI async support

   FastAPI supports async endpoints; missing async can cause blocking or warnings.

2. Step 2: Verify parameter and decorator correctness

   Parameter is correctly typed as Message; @app.post is correct for receiving data.

3. Final Answer:

   The endpoint function is missing async keyword -> Option A

4. Quick Check:

   Async keyword needed for async endpoints [OK]

Hint: Use async def for FastAPI POST endpoints [OK]

Common Mistakes:

• Using sync def without async in async context
• Wrong HTTP method decorator
• Incorrect parameter type annotation

5. You want to create a FastAPI endpoint that receives a list of messages, each with a text field, and returns a list of all texts in uppercase. Which code snippet correctly implements this?

hard

A. from fastapi import FastAPI
from pydantic import BaseModel
from typing import List

app = FastAPI()

class Message(BaseModel):
text: str

@app.get('/batch')
def batch_messages(msgs: List[Message]):
return [msg.text.upper() for msg in msgs]

B. from fastapi import FastAPI
from pydantic import BaseModel

app = FastAPI()

class Message(BaseModel):
text: str

@app.post('/batch')
async def batch_messages(msgs: Message):
return [msg.text.upper() for msg in msgs]

C. from fastapi import FastAPI
from pydantic import BaseModel
from typing import List

app = FastAPI()

class Message(BaseModel):
text: str

@app.post('/batch')
async def batch_messages(msgs: List[Message]):
return [msg.text.upper() for msg in msgs]

D. from fastapi import FastAPI
from pydantic import BaseModel
from typing import List

app = FastAPI()

class Message(BaseModel):
text: str

@app.post('/batch')
async def batch_messages(msgs: List[str]):
return [text.upper() for text in msgs]

Solution

1. Step 1: Use List[Message] to receive multiple messages

   Typing msgs as List[Message] allows FastAPI to parse a list of message objects.

2. Step 2: Return uppercase texts from each message

   List comprehension extracts each msg.text and converts it to uppercase correctly.

3. Final Answer:

   Code snippet with List[Message] parameter and async POST endpoint -> Option C

4. Quick Check:

   List of Message + async POST + uppercase list [OK]

Hint: Use List[Message] and async POST for batch message input [OK]

Common Mistakes:

• Using single Message instead of list
• Using GET instead of POST for sending data
• Typing list as List[str] instead of List[Message]