Multiple response types in FastAPI - Mini Project: Build & Apply

Multiple Response Types with FastAPI

📖 Scenario: You are building a simple web API that can return different types of responses depending on the request. Sometimes it returns plain text, sometimes JSON data.

🎯 Goal: Create a FastAPI app with one endpoint /response that returns either plain text or JSON depending on a query parameter.

📋 What You'll Learn

Create a FastAPI app instance called app

Add a GET endpoint /response with a query parameter type

If type is text, return plain text response

If type is json, return JSON response with a dictionary

Use correct FastAPI response types for each case

💡 Why This Matters

🌍 Real World

APIs often need to return different formats like JSON or plain text depending on client needs.

💼 Career

Knowing how to handle multiple response types is important for backend developers building flexible APIs.

1

Create FastAPI app instance

Create a FastAPI app instance called app by importing FastAPI and initializing it.

FastAPI

from fastapi import FastAPI

# Create FastAPI app instance called app
# Your code here

Hint

Use FastAPI() to create the app instance and assign it to app.

2

Add GET endpoint with query parameter

Add a GET endpoint /response to the app that accepts a query parameter called type of type str.

FastAPI

from fastapi import FastAPI

app = FastAPI()

# Add GET endpoint /response with query parameter type: str
# Your code here

Hint

Use @app.get("/response") decorator and define an async function with type: str parameter.

3

Return plain text or JSON based on query parameter

Inside the get_response function, return plain text 'This is plain text' if type equals 'text'. Otherwise, return a JSON dictionary {"message": "This is JSON"}.

FastAPI

from fastapi import FastAPI

app = FastAPI()

@app.get("/response")
async def get_response(type: str):
    # Return plain text if type is 'text', else return JSON
    # Your code here

Hint

Use an if statement to check type and return the correct response.

4

Use correct response classes for text and JSON

Import PlainTextResponse from fastapi.responses and update the get_response function to return PlainTextResponse with the text when type is 'text'. Return the JSON dictionary directly otherwise.

FastAPI

from fastapi import FastAPI
from fastapi.responses import PlainTextResponse

app = FastAPI()

@app.get("/response")
async def get_response(type: str):
    # Return PlainTextResponse for text type
    # Your code here

Hint

Use PlainTextResponse to send plain text responses properly in FastAPI.

Practice

1. What is the main purpose of using multiple response types in a FastAPI route?

easy

A. To speed up the API response time by caching

B. To allow the API to send different response formats based on conditions

C. To restrict the API to only one response format

D. To automatically generate database schemas

Solution

1. Step 1: Understand response types in FastAPI

   Multiple response types let an API send different formats like JSON or HTML depending on the request or logic.

2. Step 2: Identify the purpose

   This flexibility helps clients get the response format they expect or need.

3. Final Answer:

   To allow the API to send different response formats based on conditions -> Option B

4. Quick Check:

   Multiple response types = different formats [OK]

Hint: Multiple response types mean flexible output formats [OK]

Common Mistakes:

• Thinking it speeds up response time
• Believing it restricts to one format
• Confusing with database schema generation

2. Which of the following is the correct way to declare multiple response types in a FastAPI route decorator?

easy

A. @app.get('/items', responses={200: {'content': {'application/json': {}}}})

B. @app.get('/items', response_formats=['json', 'html'])

C. @app.get('/items', response_type=['json', 'html'])

D. @app.get('/items', responses={200: {'content': {'application/json': {}, 'text/html': {}}}})

Solution

1. Step 1: Review FastAPI response declaration syntax

   FastAPI uses the 'responses' parameter with status codes and content types as keys.

2. Step 2: Identify correct syntax for multiple types

   @app.get('/items', responses={200: {'content': {'application/json': {}, 'text/html': {}}}}) correctly shows multiple content types under the 200 status code.

3. Final Answer:

   @app.get('/items', responses={200: {'content': {'application/json': {}, 'text/html': {}}}}) -> Option D

4. Quick Check:

   responses param with content dict = @app.get('/items', responses={200: {'content': {'application/json': {}, 'text/html': {}}}}) [OK]

Hint: Use 'responses' with content dict for multiple types [OK]

Common Mistakes:

• Using non-existent parameters like response_type
• Declaring only one content type when multiple needed
• Incorrect dictionary structure in responses

3. Given this FastAPI route, what will be the response content type if return HTMLResponse(content='Hello') is used?

from fastapi import FastAPI
from fastapi.responses import JSONResponse, HTMLResponse

app = FastAPI()

@app.get('/greet', responses={200: {'content': {'application/json': {}, 'text/html': {}}}})
async def greet(html: bool = False):
    if html:
        return HTMLResponse(content='Hello')
    return JSONResponse(content={'message': 'Hello'})

medium

A. text/html

B. application/json

C. application/xml

D. text/plain

Solution

1. Step 1: Check the return type when html=True

   The function returns HTMLResponse with content 'Hello' when html is True.

2. Step 2: Identify the content type of HTMLResponse

   HTMLResponse sets the content type to 'text/html'.

3. Final Answer:

   text/html -> Option A

4. Quick Check:

   HTMLResponse = text/html [OK]

Hint: HTMLResponse means content type text/html [OK]

Common Mistakes:

• Assuming JSONResponse is always returned
• Confusing content type with return value
• Ignoring the condition that triggers HTMLResponse

4. Identify the error in this FastAPI route that tries to return multiple response types:

from fastapi import FastAPI
from fastapi.responses import JSONResponse, PlainTextResponse

app = FastAPI()

@app.get('/data', responses={200: {'content': {'application/json': {}, 'text/plain': {}}}})
async def data(response_type: str):
    if response_type == 'json':
        return JSONResponse(content={'key': 'value'})
    else:
        return 'Just plain text'

medium

A. responses dictionary is incorrectly formatted

B. Missing import for PlainTextResponse

C. Returning a plain string instead of a Response object for plain text

D. Function should not be async

Solution

1. Step 1: Check return types in the function

   When response_type is not 'json', the function returns a plain string instead of a Response object.

2. Step 2: Understand FastAPI response requirements

   FastAPI expects a Response object like PlainTextResponse for non-JSON responses to set headers correctly.

3. Final Answer:

   Returning a plain string instead of a Response object for plain text -> Option C

4. Quick Check:

   Plain string return needs PlainTextResponse [OK]

Hint: Return Response objects, not raw strings for non-JSON [OK]

Common Mistakes:

• Returning raw strings instead of Response objects
• Ignoring response content type headers
• Assuming async is not needed

5. You want a FastAPI route to return JSON by default but HTML if a query parameter ?format=html is given. Which approach correctly implements this with multiple response types?

from fastapi import FastAPI, Request
from fastapi.responses import JSONResponse, HTMLResponse

app = FastAPI()

@app.get('/info', responses={200: {'content': {'application/json': {}, 'text/html': {}}}})
async def info(request: Request):
    format = request.query_params.get('format')
    if format == 'html':
        # What to return here?
    else:
        return JSONResponse(content={'info': 'data'})

hard

A. return HTMLResponse(content='Info')

B. return {'info': 'Info'}

C. return JSONResponse(content={'info': 'Info'})

D. return Response(content='Info', media_type='application/json')

Solution

1. Step 1: Check the condition for HTML format

   If the query parameter 'format' equals 'html', the route should return an HTML response.

2. Step 2: Use the correct response class for HTML

   HTMLResponse sets the content type to 'text/html' and sends HTML content properly.

3. Final Answer:

   return HTMLResponse(content='<h1>Info</h1>') -> Option A

4. Quick Check:

   HTMLResponse for HTML output = return HTMLResponse(content='<h1>Info</h1>') [OK]

Hint: Use HTMLResponse to send HTML content explicitly [OK]

Common Mistakes:

• Returning dict with HTML string instead of HTMLResponse
• Using JSONResponse with HTML content type
• Setting wrong media_type in Response