code: def fetch_token(self, **kwargs):
          
          kwargs.setdefault('client_secret', self.client_config['client_secret'])
          return self.oauth2session.fetch_token(
              self.client_config['token_uri'], **kwargs)
docs: 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.