code: def call_json(self, req_json, props=None):
          
          try:
              req = json.loads(req_json)
          except:
              msg = "Unable to parse JSON: %s" % req_json
              return json.dumps(err_response(None, -32700, msg))
          return json.dumps(self.call(req, props))
docs: Deserializes req_json as JSON, invokes self.call(), and serializes result to JSON.
          Returns JSON encoded string.
  
          :Parameters:
            req_json
              JSON-RPC request serialized as JSON string
            props
              Application defined properties to set on RequestContext for use with filters. 
              For example: authentication headers.  Must be a dict.