def _refresh(self, _):
        """Refresh self.access_token.

        Args:
          _: (ignored) A function matching httplib2.Http.request's signature.
        """
        # pylint: disable=import-error
        from google.appengine.api import app_identity
        try:
            token, _ = app_identity.get_access_token(self._scopes)
        except app_identity.Error as e:
            raise exceptions.CredentialsError(str(e))
        self.access_token = token
Refresh self.access_token.

        Args:
          _: (ignored) A function matching httplib2.Http.request's signature.