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.

def from_json(cls, json_doc):
        """
        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.
        """
        d = json.loads(json_doc)
        token = cls()
        token.__dict__.update(d)
        return token