Multiple response types in FastAPI

Introduction

Sometimes, your API needs to send different kinds of answers depending on the situation. Using multiple response types lets your app do that clearly and safely.

When your API might return either data or an error message.

When you want to send JSON for success but plain text for errors.

When different clients expect different formats from the same endpoint.

When you want to clearly document all possible responses your API can send.

Syntax

FastAPI

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

app = FastAPI()

@app.get("/item/{item_id}", responses={
    200: {"content": {"application/json": {}}},
    404: {"content": {"text/plain": {}}}
})
async def read_item(item_id: int):
    if item_id == 1:
        return JSONResponse(content={"item_id": item_id, "name": "Item One"})
    else:
        return PlainTextResponse("Item not found", status_code=404)

Use the responses parameter in the route decorator to declare possible response types.

Return the appropriate response class like JSONResponse or PlainTextResponse inside the function.

Examples

This example returns JSON if success is true, otherwise plain text with an error code.

FastAPI

from fastapi.responses import JSONResponse, PlainTextResponse

@app.get("/status")
async def get_status(success: bool):
    if success:
        return JSONResponse(content={"status": "ok"})
    else:
        return PlainTextResponse("Failed to get status", status_code=400)

This endpoint returns JSON data when ready, or plain text with status 202 if still processing.

FastAPI

@app.get("/data", responses={
    200: {"content": {"application/json": {}}},
    202: {"content": {"text/plain": {}}}
})
async def get_data(ready: bool):
    if ready:
        return {"data": [1, 2, 3]}
    else:
        return PlainTextResponse("Data processing", status_code=202)

Sample Program

This FastAPI app has one endpoint that returns user info as JSON if found, or a plain text error if not.

FastAPI

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

app = FastAPI()

@app.get("/user/{user_id}", responses={
    200: {"content": {"application/json": {}}},
    404: {"content": {"text/plain": {}}}
})
async def get_user(user_id: int):
    users = {1: "Alice", 2: "Bob"}
    if user_id in users:
        return JSONResponse(content={"user_id": user_id, "name": users[user_id]})
    else:
        return PlainTextResponse("User not found", status_code=404)

Important Notes

Always specify the responses dict to help API docs show all response types.

Use response classes from fastapi.responses to control content type and status code.

Returning plain dicts defaults to JSON response automatically.

Summary

Multiple response types let your API send different formats based on conditions.

Use responses in the route decorator to declare them.

Return the right response class inside your function to match the declared types.

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