'Ensure the reference contains the specified attributes.
Raise a ValidationError if any of the given attributes is not present'

def _require_attributes(self, ref, attrs):

    missing_attrs = [attribute for attribute in attrs if self._attribute_is_empty(ref, attribute)]
    if missing_attrs:
        msg = (_('%s    field(s)    cannot    be    empty') % ',    '.join(missing_attrs))
        raise exception.ValidationError(message=msg)
