python 39 lines · 5 steps

Content negotiation in FastAPI

One endpoint serves the same report as JSON or CSV based on the client's Accept header.

Explained by highlit

FastAPI Intermediate content-negotiation dependency-injection http-headers streaming-response

1import csv
2import io
3from typing import Annotated
4 
5from fastapi import APIRouter, Depends, Header, Response
6from fastapi.responses import JSONResponse, StreamingResponse
7 
8router = APIRouter()
9 
10 
11def negotiate_media_type(accept: Annotated[str, Header()] = "application/json") -> str:
12    if "text/csv" in accept:
13        return "text/csv"
14    return "application/json"
15 
16 
17def rows_to_csv(rows: list[dict]) -> str:
18    buffer = io.StringIO()
19    writer = csv.DictWriter(buffer, fieldnames=list(rows[0].keys()))
20    writer.writeheader()
21    writer.writerows(rows)
22    return buffer.getvalue()
23 
24 
25@router.get("/reports/daily-sales")
26async def daily_sales(
27    media_type: Annotated[str, Depends(negotiate_media_type)],
28    sales_repo: Annotated[SalesRepository, Depends(get_sales_repo)],
29) -> Response:
30    rows = await sales_repo.aggregate_by_day()
31 
32    if media_type == "text/csv":
33        return StreamingResponse(
34            iter([rows_to_csv(rows)]),
35            media_type="text/csv",
36            headers={"Content-Disposition": "attachment; filename=daily-sales.csv"},
37        )
38 
39    return JSONResponse(content={"results": rows})

01 / 01

STEP 01

Walkthrough

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Three takeaways

1Reading the Accept header lets a single endpoint serve multiple representations of the same data.
2Packaging negotiation logic in a dependency keeps the route handler focused on producing data.
3StreamingResponse with a Content-Disposition header turns generated text into a downloadable file.

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