What response types can you return beyond JSON?

Question

IT Interview Practice · Accepted Answer

FastAPI returns JSON by default (serializing dicts, lists, and Pydantic models), but it supports many other **response types** for different needs — HTML, files, streams, redirects, and plain text — via the `fastapi.responses` classes. ## The default and common response classes ```python from fastapi.responses import ( JSONResponse, HTMLResponse, PlainTextResponse, RedirectResponse, FileResponse, StreamingResponse, ) @app.get("/json") def j(): return {"key": "value"} # default → JSON @app.get("/html", response_class=HTMLResponse) def h(): return "

Hello

" # serve HTML @app.get("/text", response_class=PlainTextResponse) def t(): return "plain text" @app.get("/go") def g(): return RedirectResponse(url="/new-location") # 307 redirect ``` ## Serving files ```python @app.get("/download") def download(): return FileResponse( "report.pdf", media_type="application/pdf", filename="report.pdf", # prompts a download with this name ) ``` `FileResponse` efficiently serves a file from disk (handling headers, content type) — for downloads, images, generated documents. ## Streaming large or generated data ```python @app.get("/stream") def stream(): def generate(): for i in range(1000000): yield f"line {i} " # yield chunks — NOT all loaded into memory return StreamingResponse(generate(), media_type="text/plain") ``` `StreamingResponse` sends data incrementally from a generator — essential for **large or continuously-generated** data (big files, CSV exports, live feeds) so you don't load it all into memory at once. (Also the basis of Server-Sent Events.) ## Custom JSON encoders (faster JSON) ```python from fastapi.responses import ORJSONResponse app = FastAPI(default_response_class=ORJSONResponse) # use orjson for faster JSON serialization ``` ## Why it matters While JSON covers most API responses, real applications often need to return other content — and knowing FastAPI's response types lets you handle these correctly. **`FileResponse`** is needed for serving files and downloads (reports, images, generated documents), **`StreamingResponse`** is important for **memory-efficient** delivery of large or continuously-generated data (big exports, file streaming, server-sent events — yielding chunks rather than buffering everything in memory, which matters for scalability), and `HTMLResponse`/`PlainTextResponse`/`RedirectResponse` handle non-JSON content and navigation. Understanding the available response types and especially when to **stream** (for large data, to avoid memory exhaustion) versus return JSON is practical knowledge for building APIs that go beyond simple JSON endpoints — covering file downloads, data exports, and large or real-time responses that are common requirements in real-world applications.

The default and common response classes

Serving files

Streaming large or generated data

Custom JSON encoders (faster JSON)

Why it matters

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