FastAPI singleton service dependency injection across multiple requests

application.py

import random

import click
import uvicorn
from fastapi import FastAPI, APIRouter, Depends, Request


# Service
class MySingletonService:
    def __init__(self):
        self.random_number = random.randint(1, 100)

    def __str__(self):
        return 'my_singleton_service'

    def say_hello(self) -> str:
        return f'Hello Python Community! You will see "{self.random_number}" for every request.'


# Helper to grab dependencies that live in the app.state
def get_my_singleton_service(request: Request) -> MySingletonService:
    # See application for the key name in `app`.
    return request.app.state.my_singleton_service


# Router
router = APIRouter(prefix='/linkedin', tags=['LinkedIn'])


@router.get('/hello_python', response_model=str)
async def edge_detection(my_singleton_service: MySingletonService = Depends(get_my_singleton_service)) -> str:
    return my_singleton_service.say_hello()


# FastAPI Application Factory
def create_app() -> FastAPI:
    """Creates the FastAPI application.
    """
    application = FastAPI(title='This is single-instance DI across requests!')
    application.include_router(router=router)

    # Register our singleton service for DI once when the app has finished startup
    @application.on_event('startup')
    def load_single_instance_service():
        application.state.my_singleton_service = MySingletonService()

    return application


@click.command()
@click.option('--host', default='127.0.0.1', help='host')
@click.option('--port', default='31313', help='port')
@click.option('--hot-reload', is_flag=True, default=False, help='Hot reload')
@click.option('--log-level', default='info', help='Set the log level. Default: "info".')
def main(host, port, hot_reload, log_level):
    uvicorn.run('application:create_app', host=host, port=int(port), reload=hot_reload, log_level=log_level, factory=True)


if __name__ == '__main__':
    main()