:type item: moto.dynamodb2.models.Item

def _lhs(self, item):
        """
        :type item: moto.dynamodb2.models.Item
        """
        lhs = self.lhs
        if isinstance(self.lhs, (Op, Func)):
            lhs = self.lhs.expr(item)
        elif isinstance(self.lhs, six.string_types):
            try:
                lhs = item.attrs[self.lhs].cast_value
            except Exception:
                pass

        return lhs