def map(self, map_fn, desc=None):
        """
        Return a copy of this query, with the values mapped through `map_fn`.

        Args:
            map_fn (callable): A callable that takes a single argument and returns a new value.

        Keyword Args:
            desc (str): A description of the mapping transform, for use in log message.
                Defaults to the name of the map function.

        Returns:
            Query
        """
        if desc is None:
            desc = getattr(map_fn, '__name__', '')
        desc = u'map({})'.format(desc)

        return self.transform(lambda xs: (map_fn(x) for x in xs), desc=desc)
Return a copy of this query, with the values mapped through `map_fn`.

        Args:
            map_fn (callable): A callable that takes a single argument and returns a new value.

        Keyword Args:
            desc (str): A description of the mapping transform, for use in log message.
                Defaults to the name of the map function.

        Returns:
            Query