Get the expected values for the update

def expected(self, dynamizer):
        """ Get the expected values for the update """
        if self._expect_kwargs:
            return encode_query_kwargs(dynamizer, self._expect_kwargs)
        if self._expected is not NO_ARG:
            ret = {}
            if is_null(self._expected):
                ret['Exists'] = False
            else:
                ret['Value'] = dynamizer.encode(self._expected)
                ret['Exists'] = True
            return {self.key: ret}
        return {}