code: def from_json(cls, json_doc):
          
          d = json.loads(json_doc)
          token = cls()
          token.__dict__.update(d)
          return token
docs: Create and return a new Session Token based on the contents
          of a JSON document.
  
          :type json_doc: str
          :param json_doc: A string containing a JSON document with a
              previously saved Credentials object.