Multiple response types in FastAPI - Step-by-Step Execution

Concept Flow - Multiple response types

Client sends request

↓

FastAPI endpoint receives

↓

Check request details

↓

Decide response type

↓

Return JSON

↓

Client receives response

FastAPI checks the request and decides which response type to send back, like JSON or HTML.

Execution Sample

FastAPI

from fastapi import FastAPI
from fastapi.responses import HTMLResponse

app = FastAPI()

@app.get("/", response_class=HTMLResponse)
async def root():
    return "<h1>Hello HTML</h1>"

This code sends an HTML response when the root URL is accessed.

Execution Table

Step	Action	Request Type	Response Type	Returned Content
1	Client sends GET /	GET /	Check endpoint	None
2	FastAPI matches route /	GET /	Determine response_class	HTMLResponse
3	Call root() function	GET /	Prepare HTML content	"<h1>Hello HTML</h1>"
4	Send response to client	GET /	HTMLResponse	<h1>Hello HTML</h1>
5	Client receives response	GET /	HTML content	Page shows 'Hello HTML' in heading

💡 Response sent and client received the HTML content

Variable Tracker

Variable	Start	After Step 3	Final
response_class	None	HTMLResponse	HTMLResponse
returned_content	None	"<h1>Hello HTML</h1>"	"<h1>Hello HTML</h1>"

Key Moments - 2 Insights

Why does FastAPI send HTML instead of plain text here?

Because the endpoint uses response_class=HTMLResponse, FastAPI knows to send HTML, as shown in execution_table step 2.

What happens if we don't specify response_class?

FastAPI defaults to JSONResponse for dict returns and PlainTextResponse for string returns, so it would send plain text instead of HTML. This is implied by the check in step 2.

Visual Quiz - 3 Questions

Look at the execution table, what is the response_class at step 2?

A HTMLResponse

B JSONResponse

C PlainTextResponse

D None

💡 Hint

Check the 'Response Type' column at step 2 in the execution_table

At which step does FastAPI prepare the HTML content to send?

A Step 1

B Step 4

C Step 3

D Step 5

💡 Hint

Look at the 'Returned Content' column in the execution_table

If we remove response_class=HTMLResponse, what would change in the execution_table?

A Client would receive HTML content

B Response Type at step 2 would be PlainTextResponse

C Returned Content would be HTML

D No response would be sent

💡 Hint

Refer to key_moments about default response type when response_class is not set

Concept Snapshot

FastAPI endpoints can return different response types.
Use response_class parameter to specify type like HTMLResponse.
Default is PlainTextResponse if not set for string returns.
This controls how data is sent to the client.
Helps serve JSON, HTML, plain text, or files easily.

Full Transcript

This visual trace shows how FastAPI handles multiple response types. When a client sends a request, FastAPI matches the route and checks the response_class. If HTMLResponse is set, it calls the endpoint function and prepares HTML content. Then it sends this HTML back to the client. Variables like response_class and returned_content change during execution. Beginners often wonder why HTML is sent instead of plain text; it's because of the response_class setting. If not set, FastAPI defaults to PlainTextResponse for string returns. This trace helps understand how FastAPI decides what to send back.