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)

do you know if it can handle post requests?

Not like this.
You need to override the post() method of the resuest handler to support post queries.

See the doc of tornado :
https://www.tornadoweb.org/en/stable/web.html