Why is my FastAPI StreamingResponse not streaming with a generator function?

FastAPI StreamingResponse Not Streaming with Generator Function

Problem:

A FastAPI application fails to stream a response from a generator function using StreamingResponse, resulting in the entire response being sent as a whole.

Answer:

There are several factors to consider when using StreamingResponse with generator functions:

1. HTTP Request Type:

The provided code uses a POST request, which is not suitable for obtaining data from the server. Use a GET request instead to fetch data.

2. Credential Handling:

For security reasons, avoid sending credentials (e.g., 'auth_key') via the URL query string. Use headers or cookies instead.

3. Generator Function Syntax:

Blocking operations should not be executed within the generator function of StreamingResponse. Use def instead of async def for the generator function, as FastAPI uses a thread pool to manage blocking operations.

4. Iterator Usage:

In your test code, requests.iter_lines() iterates through the response data one line at a time. If the response does not contain line breaks, use iter_content() and specify a chunk size to avoid potential buffering issues.

5. Media Type:

Browsers may buffer responses with media_type='text/plain'. To avoid this, set media_type='text/event-stream' or disable MIME Sniffing using X-Content-Type-Options: nosniff in the response headers.

Working Example:

Here is a working example in app.py and test.py that addresses the issues mentioned above:

# app.py

from fastapi import FastAPI, StreamingResponse
import asyncio

app = FastAPI()

async def fake_data_streamer():
    for i in range(10):
        yield b'some fake data\n\n'
        await asyncio.sleep(0.5)

@app.get('/')
async def main():
    headers = {'X-Content-Type-Options': 'nosniff'}  # Disable MIME Sniffing
    return StreamingResponse(fake_data_streamer(), media_type='text/event-stream', headers=headers)

# test.py (using httpx)

import httpx

url = 'http://localhost:8000/'

with httpx.stream('GET', url) as r:
    for chunk in r.iter_content(1024):
        print(chunk)

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