def fetch_token(self, **kwargs):
        """Completes the Authorization Flow and obtains an access token.

        This is the final step in the OAuth 2.0 Authorization Flow. This is
        called after the user consents.

        This method calls
        :meth:`requests_oauthlib.OAuth2Session.fetch_token`
        and specifies the client configuration's token URI (usually Google's
        token server).

        Args:
            kwargs: Arguments passed through to
                :meth:`requests_oauthlib.OAuth2Session.fetch_token`. At least
                one of ``code`` or ``authorization_response`` must be
                specified.

        Returns:
            Mapping[str, str]: The obtained tokens. Typically, you will not use
                return value of this function and instead and use
                :meth:`credentials` to obtain a
                :class:`~google.auth.credentials.Credentials` instance.
        """
        kwargs.setdefault('client_secret', self.client_config['client_secret'])
        return self.oauth2session.fetch_token(
            self.client_config['token_uri'], **kwargs)
Completes the Authorization Flow and obtains an access token.

        This is the final step in the OAuth 2.0 Authorization Flow. This is
        called after the user consents.

        This method calls
        :meth:`requests_oauthlib.OAuth2Session.fetch_token`
        and specifies the client configuration's token URI (usually Google's
        token server).

        Args:
            kwargs: Arguments passed through to
                :meth:`requests_oauthlib.OAuth2Session.fetch_token`. At least
                one of ``code`` or ``authorization_response`` must be
                specified.

        Returns:
            Mapping[str, str]: The obtained tokens. Typically, you will not use
                return value of this function and instead and use
                :meth:`credentials` to obtain a
                :class:`~google.auth.credentials.Credentials` instance.