code: def credentials_from_session(session, client_config=None):
      
      client_config = client_config if client_config is not None else {}
  
      if not session.token:
          raise ValueError(
              'There is no access token for this session, did you call '
              'fetch_token?')
  
      credentials = google.oauth2.credentials.Credentials(
          session.token['access_token'],
          refresh_token=session.token.get('refresh_token'),
          id_token=session.token.get('id_token'),
          token_uri=client_config.get('token_uri'),
          client_id=client_config.get('client_id'),
          client_secret=client_config.get('client_secret'),
          scopes=session.scope)
      credentials.expiry = datetime.datetime.utcfromtimestamp(
          session.token['expires_at'])
      return credentials
docs: Creates :class:`google.oauth2.credentials.Credentials` from a
      :class:`requests_oauthlib.OAuth2Session`.
  
      :meth:`fetch_token` must be called on the session before before calling
      this. This uses the session's auth token and the provided client
      configuration to create :class:`google.oauth2.credentials.Credentials`.
      This allows you to use the credentials from the session with Google
      API client libraries.
  
      Args:
          session (requests_oauthlib.OAuth2Session): The OAuth 2.0 session.
          client_config (Mapping[str, Any]): The subset of the client
              configuration to use. For example, if you have a web client
              you would pass in `client_config['web']`.
  
      Returns:
          google.oauth2.credentials.Credentials: The constructed credentials.
  
      Raises:
          ValueError: If there is no access token in the session.