code: def get_json_field(self, field, **kwargs):
          
          d = self.request(
              'GET', headers={'Accept': 'application/json'}, **kwargs)
          d.addCallback(raise_for_status)
          d.addCallback(raise_for_header, 'Content-Type', 'application/json')
          d.addCallback(json_content)
          d.addCallback(self._get_json_field, field)
          return d
docs: Perform a GET request and get the contents of the JSON response.
  
          Marathon's JSON responses tend to contain an object with a single key
          which points to the actual data of the response. For example /v2/apps
          returns something like {"apps": [ {"app1"}, {"app2"} ]}. We're
          interested in the contents of "apps".
  
          This method will raise an error if:
          * There is an error response code
          * The field with the given name cannot be found