Helper to get value of a named attribute irrespective of whether it is passed
        with or without "@" prefix.

def _get_kwarg(self, name, kwargs):
        """
        Helper to get value of a named attribute irrespective of whether it is passed
        with or without "@" prefix.
        """
        at_name = '@{}'.format(name)

        if name in kwargs:
            if at_name in kwargs:
                raise ValueError('Both {!r} and {!r} specified in kwargs'.format(name, at_name))
            return kwargs[name]

        if at_name in kwargs:
            return kwargs[at_name]

        return not_set