How do you stream large responses in FastAPI using StreamingResponse?

StreamingResponse lets you send data incrementally — essential for large file downloads, CSV exports, real-time data feeds, or AI token streaming — without loading everything into memory first.

from fastapi import FastAPI
from fastapi.responses import StreamingResponse
import asyncio
import csv
import io

app = FastAPI()

# Stream a large CSV without loading it all into memory
@app.get("/export/users")
async def export_users():
    async def generate_csv():
        output = io.StringIO()
        writer = csv.writer(output)
        writer.writerow(["id", "username", "email"])  # header
        yield output.getvalue()
        output.seek(0)
        output.truncate(0)

        for i in range(1_000_000):  # stream 1M rows
            writer.writerow([i, f"user{i}", f"user{i}@example.com"])
            if i % 1000 == 0:  # flush every 1000 rows
                yield output.getvalue()
                output.seek(0)
                output.truncate(0)
                await asyncio.sleep(0)  # yield control to event loop

    return StreamingResponse(
        generate_csv(),
        media_type="text/csv",
        headers={"Content-Disposition": "attachment; filename=users.csv"},
    )

# Server-Sent Events (SSE) for real-time updates
@app.get("/events")
async def event_stream():
    async def generate():
        for i in range(10):
            yield f"data: Event {i}\n\n"  # SSE format
            await asyncio.sleep(1)

    return StreamingResponse(
        generate(),
        media_type="text/event-stream",
        headers={"Cache-Control": "no-cache"},
    )

# Stream a file from disk
@app.get("/download/{filename}")
async def download_file(filename: str):
    def iterfile():
        with open(f"/data/{filename}", "rb") as f:
            yield from f  # yields chunks as they are read

    return StreamingResponse(iterfile(), media_type="application/octet-stream")

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