Sets key/values based on a mapping if none of the keys are already set.
        Mapping is a dictionary of key/value pairs. Both keys and values
        should be strings or types that can be cast to a string via str().
        Returns a boolean indicating if the operation was successful.

def msetnx(self, mapping):
        """
        Sets key/values based on a mapping if none of the keys are already set.
        Mapping is a dictionary of key/value pairs. Both keys and values
        should be strings or types that can be cast to a string via str().
        Returns a boolean indicating if the operation was successful.
        """
        items = []
        for pair in iteritems(mapping):
            items.extend(pair)
        return self.execute_command('MSETNX', *items)