Add custom rest API to streamlit app

api.py

# This code adds custom REST api handler at runtime to a running Streamlit app
#

from tornado.web import Application, RequestHandler
from tornado.routing import Rule, PathMatches
import gc
import streamlit as st


@st.cache_resource()
def setup_api_handler(uri, handler):
    print("Setup Tornado. Should be called only once")

    # Get instance of Tornado
    tornado_app = next(o for o in gc.get_referrers(Application) if o.__class__ is Application)

    # Setup custom handler
    tornado_app.wildcard_router.rules.insert(0, Rule(PathMatches(uri), handler))
    
    
    
# Usage 
class HelloHandler(RequestHandler):
  def get(self):
    self.write({'message': 'hello world'})

# This setup will be run only once
setup_api_handler('/api/hello', HelloHandler)

Thanks! I was looking for that feature in Streamlit apps.
I just want to add that if one wishes to get the argument of the request, here's how to do it:
e.g. the URI is http://smthng/api/hello?my_arg=hi
you can access the value by using self.get_argument("my_arg", "default value")