server.py

import json

import tornado.ioloop
import tornado.web

DB = dict()

class BaseHandler(tornado.web.RequestHandler):
    def validate_input(self):
        if not isinstance(self.json_args.get('key'), str):
            self.write(dict(success=False, reason="key must be a string"))

    def prepare(self):
        if self.request.headers.get("Content-Type", "") == "application/json":
            try:
                self.json_args = json.loads(self.request.body)
            except json.decoder.JSONDecodeError as e:
                # TODO return error message
                self.json_args = None
            else:
                self.validate_input()
        else:
            self.json_args = None

class ResourceHandler(BaseHandler):
    def get(self, key):
        self.write(json.dumps(DB[key]))

class MainHandler(BaseHandler):
    def get(self):
        self.write(json.dumps(DB))

    def post(self):
        key = self.json_args.get('key')
        value = self.json_args.get('value')
        if key not in DB:
            DB[key] = value
            success = True
        else:
            success = False
        self.write(json.dumps(dict(success=success)))

    def put(self):
        key = self.json_args.get('key')
        value = self.json_args.get('value')
        if key in DB:
            DB[key] = value
            success = True
        else:
            success = False
        self.write(json.dumps(dict(success=success)))

    def delete(self):
        key = self.json_args.get('key')
        if key in DB:
            del DB[key]
            success = True
        else:
            success = False
        self.write(json.dumps(dict(success=success)))

def make_app():
    print("starting server")
    return tornado.web.Application([
        (r"/", MainHandler),
        (r"/([a-zA-Z0-9]+)", ResourceHandler),
    ],
    debug=True)

if __name__ == "__main__":
    app = make_app()
    app.listen(8888)
    tornado.ioloop.IOLoop.current().start()

you'll need to install tornado - guide here: http://www.tornadoweb.org/en/stable/guide/structure.html