Can param _validate update input value?

class Cartopy(param.ClassSelector):
    def __init__(self, default=None, **params):
        import cartopy.crs as ccrs
        objects = tuple(
            obj for name, obj in vars(ccrs).items()
            if isinstance(obj, type) and issubclass(obj, ccrs.Projection) and
            not name.startswith('_') and name not in ['Projection']
        )
        super(Cartopy, self).__init__(objects, **params)
        self._validate(self.default)

    def _validate():
    # if input value is not in listed values, return ccrs.PlateCarree()