How to use the gwcs.utils.isnumerical function in gwcs

The analytical inverse of the forward transform is used, if available.
        If not an iterative method is used.

        Parameters
        ----------
        args : float, array like, `~astropy.coordinates.SkyCoord` or `~astropy.units.Unit`
            coordinates to be inverted
        kwargs : dict
            keyword arguments to be passed to the iterative invert method.
        with_bounding_box : bool, optional
             If True(default) values in the result which correspond to any of the inputs being
             outside the bounding_box are set to ``fill_value``.
        fill_value : float, optional
            Output value for inputs outside the bounding_box (default is np.nan).
        """
        if not utils.isnumerical(args[0]):
            args = self.output_frame.coordinate_to_quantity(*args)
            if self.output_frame.naxes == 1:
                args = [args]
            if not self.backward_transform.uses_quantity:
                args = utils.get_values(self.output_frame.unit, *args)

        with_units = kwargs.pop('with_units', False)
        if 'with_bounding_box' not in kwargs:
            kwargs['with_bounding_box'] = True
        if 'fill_value' not in kwargs:
            kwargs['fill_value'] = np.nan

        try:
            result = self.backward_transform(*args, **kwargs)
        except (NotImplementedError, KeyError):
            result = self._invert(*args, **kwargs)

def world_to_pixel(self, *world_objects):
        """
        Convert world coordinates to pixel values.
        """
        result = self.invert(*world_objects, with_units=True)
        if not utils.isnumerical(result[0]):
            result = [i.value for i in result]
        if self.input_frame.naxes == 1:
            return result[0]
        return result

Initial coordinate frame.
        to_frame : str, or instance of `~gwcs.cordinate_frames.CoordinateFrame`
            Coordinate frame into which to transform.
        args : float or array-like
            Inputs in ``from_frame``, separate inputs for each dimension.
        output_with_units : bool
            If ``True`` - returns a `~astropy.coordinates.SkyCoord` or
            `~astropy.units.Quantity` object.
        with_bounding_box : bool, optional
             If True(default) values in the result which correspond to any of the inputs being
             outside the bounding_box are set to ``fill_value``.
        fill_value : float, optional
            Output value for inputs outside the bounding_box (default is np.nan).
        """
        transform = self.get_transform(from_frame, to_frame)
        if not utils.isnumerical(args[0]):
            inp_frame = getattr(self, from_frame)
            args = inp_frame.coordinate_to_quantity(*args)
            if not transform.uses_quantity:
                args = utils.get_values(inp_frame.unit, *args)

        with_units = kwargs.pop("with_units", False)
        if 'with_bounding_box' not in kwargs:
            kwargs['with_bounding_box'] = True
        if 'fill_value' not in kwargs:
            kwargs['fill_value'] = np.nan

        result = transform(*args, **kwargs)

        if with_units:
            to_frame_name, to_frame_obj = self._get_frame_name(to_frame)
            if to_frame_obj is not None: