How to forward OpenAI's stream response using FastAPI in python?

API
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1

This is my code to retrieve stream response from OpenAI’s model which is event based. (I have shown only core part)

client = OpenAI(api_key=OPEN_AI_API_KEY)

class EventHandler(AssistantEventHandler):
    def on_text_delta(self, delta: TextDelta, snapshot: Text):
        print(delta.value)

with client.beta.threads.runs.stream(
    thread_id=thread_id,
    assistant_id=assistant_id,
    event_handler=EventHandler()
) as stream:
stream.until_done()

on_text_delta event triggers as tokens arrives from API. I want to forward this response using FastAPI instead of printing on output screen.

@app.get("/stream")
async def stream():
    return ...something...

I have tried responding result as part of HTTP body:

from fastapi.responses import StreamingResponse

...

@app.post("/stream")
async def stream():
    with client.beta.threads.runs.stream(
        thread_id=thread_id,
        assistant_id=assistant_id,
        event_handler=EventHandler()
    ) as stream:
        stream.until_done()

    return StreamingResponse(EventHandler.generator_function(), media_type="text/plain")

I have created generator_function inside EventHandler class but problem is until stream is not over the execution doesn’t reach return statement.

I have also tried websockets, but still problem is how should my program execution should flow. The stream doesn’t let execution go further until API response is completed.

2

I found the solution!

from fastapi import FastAPI
from fastapi.responses import StreamingResponse
from openai import OpenAI, AsyncOpenAI


OPEN_AI_API_KEY = 'you_api_key'
async_client = AsyncOpenAI(api_key=OPEN_AI_API_KEY)
client = OpenAI(api_key=OPEN_AI_API_KEY)

app = FastAPI()

async def stream_assistant_response(assistant_id, thread_id):
    stream =  async_client.beta.threads.runs.stream(
        assistant_id=assistant_id,
        thread_id=thread_id
    )

    async with stream as stream:
        async for text in stream.text_deltas:
            yield f"data: {text}\n\n"


@app.get("/message")
async def add_message(assistant_id, thread_id, message):
    # make sure thread exist
    client.beta.threads.messages.create(
        thread_id=thread_id,
        role="user",
        content=message
    )

    return StreamingResponse(stream_assistant_response(assistant_id, thread_id), media_type="text/event-stream")
3

Can you let me know if this stream honored white space and new line as it gets from the stream? For me all stream is displayed into the same line.

4

@vkoprivica.education

Not sure if I completely understood your problem but white space won’t be a problem in forwarding as it is similar to doing this:

yield f"data:  \n\n" # two spaces, two \n

But yes, fowarding \n is problematic due to Server Sent Events (SSE) format which would result like this:

yield f"data: \n\n\n" # one space, three \n

To handle such situation you can do following:

 async def stream_assistant_response(assistant_id, thread_id):
    stream =  async_client.beta.threads.runs.stream(
        assistant_id=assistant_id,
        thread_id=thread_id
    )

    async with stream as stream:
        async for text in stream.text_deltas:
            formatted_text = text.replace('\n', '\\n')
            yield f"data: {formatted_text}\n\n"
6

Thank you for responding. In my case this actually worked:

FastAPI Implementation

yield f"data: {text.replace('\n', '<br>')}\n\n"

```java script
const outputDiv = document.getElementById('streamOutput');
eventSource.onmessage = (event) => {
    outputDiv.innerHTML += event.data;

```html
<pre id="streamOutput"></pre>