[Python] How to send server-side events from python (fastapi) upon calls to a function that...

I have the following problem: given a backend running fastapi, that has a streaming endpoint, which is used to update the frontend, I want to send these updates every time the function that updates the backend state gets called (which can be either by an scheduled job or a different endpoint that was hit and caused the state to be updated).

A simplistic version of what I would like to implement would be:

from fastapi import FastAPI
from starlette.responses import StreamingResponse

class State:
def __init__(self):
self.messages = []

def update(self, new_messages):
self.messages = new_messages
# HERE: notify waiting stream endpoint

app = FastAPI()

state = State()

@app.get('/stream')
def stream():
def event_stream():
while True:
# HERE lies the question: wait for state to be update
for message in state.messages:
yield 'data: {}\n\n'.format(json.dumps(message))
return StreamingResponse(event_stream(), media_type="text/event-stream")


And I would like this to keep running forever. Every time the state gets updated, the event_stream unblocks and send the messages.

I have looked a threading and asyncio, but I have a feeling that I am missing some simple concept of how to do this in python.

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