def apply(self, func, shortcut=False, args=(), **kwargs):
        """
        Backward compatible implementation of ``map``

        See Also
        --------
        DataArrayGroupBy.map
        """
        warnings.warn(
            "GroupBy.apply may be deprecated in the future. Using GroupBy.map is encouraged",
            PendingDeprecationWarning,
            stacklevel=2,
        )
        return self.map(func, shortcut=shortcut, args=args, **kwargs)

        Backward compatible implementation of ``map``

        See Also
        --------
        DataArrayGroupBy.map
        
xarray/core/groupby.py