Python and Javascript shared file on Google App Engine / Google Cloud Platform

The principle known as 'single source of truth' (SSoT) is quite important in computing, and is perhaps underappreciated. We need to write programs whose assumptions are consistent and easily discoverable. It's important to avoid the introduction of multiple sources for the same 'things': things that can be as simple as constants, and as complex as the definition of terminology within a theory of automation.

Sometimes, we need this to take place within a single shared file, accessible during runtime between the server and the client code, to express important editable values once.

I use local files in my development environment, to make changes to my webapp. In one important application, I don't need a makefile-like compiling stage. So, I test locally with a development server and a browser, as everyone does. But I want to have configuration files -- central, essential, important files -- which pretty much define everything critical in my application, both the universal and the contingent aspects ... and all the intermediate levels of detail needed to properly define those aspects of the work.

I call this an 'essence' or 'central description'. It is neither javascript (well, it's JSON here for technical convenience) nor python. It's just a configuration file. I've found that this compels me to maintain a quite useful habit: keep things modularized and parameterized at a higher level than code, making important ideas resuable as the application unfolds.

But how do you connect a Google App Engine Python server application (on Google Cloud Platform) to the javascript that it serves up? It seems like one should be able to use a shared JSON file easily. But that's not the case.

However, given that it's extremely important, here are the technical details of connecting them.

The shared file is shared.json.
The server-side python is called my_app.py.
The client-side javascript is my_app.js.
There's an app.yaml.
They are all in the same folder.
And I'm intentionally ignoring the html necessary to get to the javascript code, which could be embedded script, or a separate file, static, or generated ... that's all up to you. I don't know where you run your javascript, so I've just called it 'my_app.js'.

shared.json

{
 // a comment
 "name":"my_app",
 "things": {
 "one":"a"
 "two":"b"
 }
}


my_app.py

import webapp2
import io
import json

# this version of strip_comments 
# only works if the comment is defined
# by //, and // is the first non-whitespace
# on the line

def strip_comments(some_json):
 some_json = str(some_json)
 return re.sub(r'(?m)^[ \t]*//.*\n?', '', some_json)
    
class ReadAndRespond(webapp2.RequestHandler):
 def get(self):

 # get the file
 file = io.open("shared.json", "rb", buffering = 5)
 json_string = file.read()
 file.close()
 
 # strip the comments
 json_sans_comments = strip_comments(json_string)
 
 # load the JSON into a python dictionary
 dict_obj = json.loads(json_sans_comments)
 
 # get one of the values 
 my_things = dict_obj.get("things")
 
 # use it for a new JSON payload for the response
 # to the javascript request
 return_string = json.dumps(my_things, sort_keys=True, indent=4)

 self.response.write(return_string)

app = webapp2.WSGIApplication([('the_json', ReadAndRespond)])


my_app.js

 ...

 var fetch = eval( ajaxReturn('the_json') );
 var things = fetch;

 ...
 
 function ajaxReturn(xhr_url) {
    var return_string = '';
    $.ajax({
                dataType: "text",
                url : xhr_url,
                async: false,
                success : function (newContent) {
                   return_string = newContent;
                },
                error : function ()
                {
                }
    });
    return return_string;
 }
 
 ...

app.yaml

runtime: python27
api_version: 1
threadsafe: false

handlers:
- url: /shared.json
  static_files: shared.json
  application_readable: true
  secure: always
  redirect_http_response_code: 301
  
  - url: /.*
  script: my_app.app
  secure: always
  redirect_http_response_code: 301