FastAPI-SSE

A tiny library for sending Server-Sent Events (SSE) in FastAPI

Server-Sent Events (SSE) provide a way to stream real-time updates from the server to the client over HTTP. This library allows sending Pydantic models as SSE events in FastAPI, formatted as JSON.

from fastapi import FastAPI
from fastapi_sse import sse_response
from pydantic import BaseModel

app = FastAPI()

class MyMessage(BaseModel):
    text: str

@app.get("/stream")
@sse_handler()
async def message_generator(some_url_arg: str):
    yield MyMessage(text=f"Hello, {some_url_arg}!")
    yield MyMessage(text="Another message")

And on the frontend to handle:

const eventSource = new EventSource('/stream');
eventSource.onmessage = (event) => {
    const message = JSON.parse(event.data);
    console.log('Received message:', message);
};

Installation

Install the library using pip:

pip install fastapi-sse

Typed Events

You can also use typed_sse_response to send messages with the event type populated with the model name.

Note: When emitting typed messages, EventSource will no longer trigger the onmessage event. Instead, you must attach a handler via .addEventListener('<eventType>', ...).:

const eventSource = new EventSource('/stream');
eventSource.addEventListener('MyMessage', (event) => {
    const message = JSON.parse(event.data);
    console.log('Received message:', message);
});

Development

FastAPI-SSE uses Rye for dependency management and the development workflow. To get started with development, ensure you have Rye installed and then clone the repository and set up the environment:

git clone https://github.com/MatthewScholefield/fastapi-sse.git
cd fastapi-sse
rye sync
rye run pre-commit install

# Run tests
rye test