def assert_keys_exist(self, caller, *keys):
        """Assert that context contains keys.

        Args:
            keys: validates that these keys exists in context
            caller: string. calling function or module name - this used to
                    construct error messages

        Raises:
            KeyNotInContextError: When key doesn't exist in context.

        """
        assert keys, ("*keys parameter must be specified.")
        for key in keys:
            self.assert_key_exists(key, caller)