Stores always valid OAuth2 access token.

        Note:
            Accessing this property may result in HTTP request.

        Returns:
            str

def access_token(self):
        """Stores always valid OAuth2 access token.

        Note:
            Accessing this property may result in HTTP request.

        Returns:
            str
        """
        if (self._access_token is None or
                self.expiration_time <= int(time.time())):
            resp = self.make_access_request()
            self._access_token = resp.json()['access_token']

        return self._access_token