Preserve get_area_slices behavior when area to cover has an invalid b…

…oundary

pyresample/geometry.py

@@ -2592,14 +2592,8 @@ def get_area_slices(self, area_to_cover, shape_divisible_by=None):
                                       "equal.")
 
         data_boundary = Boundary(*get_geostationary_bounding_box_in_lonlats(self))
-        if area_to_cover.is_geostationary:
-            area_boundary = Boundary(
-                *get_geostationary_bounding_box_in_lonlats(area_to_cover))
-        else:
-            area_boundary = AreaDefBoundary(area_to_cover, 100)
-
-        intersection = data_boundary.contour_poly.intersection(
-            area_boundary.contour_poly)
+        area_boundary = self._get_area_to_cover_boundary(area_to_cover)
+        intersection = data_boundary.contour_poly.intersection(area_boundary.contour_poly)
         if intersection is None:
             logger.debug('Cannot determine appropriate slicing. '
                          "Data and projection area do not overlap.")
@@ -2619,6 +2613,15 @@ def get_area_slices(self, area_to_cover, shape_divisible_by=None):
         return (check_slice_orientation(x_slice),
                 check_slice_orientation(y_slice))
 
+    @staticmethod
+    def _get_area_to_cover_boundary(area_to_cover: AreaDefinition) -> Boundary:
+        try:
+            if area_to_cover.is_geostationary:
+                return Boundary(*get_geostationary_bounding_box_in_lonlats(area_to_cover))
+            return AreaDefBoundary(area_to_cover, 100)
+        except ValueError:
+            raise NotImplementedError("Can't determine boundary of area to cover")
+
     def crop_around(self, other_area):
         """Crop this area around `other_area`."""
         xslice, yslice = self.get_area_slices(other_area)

pyresample/test/test_geometry/test_area.py

@@ -1814,6 +1814,17 @@ def test_on_flipped_geos_area(self, create_test_area):
         assert slice_lines == expected_slice_lines
         assert slice_cols == expected_slice_cols
 
+    def test_area_to_cover_all_nan_bounds(self, geos_src_area, create_test_area):
+        """Check area slicing when the target doesn't have a valid boundary."""
+        area_def = geos_src_area
+        # An area that is a subset of the original one
+        area_to_cover = create_test_area(
+            {"proj": "moll"},
+            1000, 1000,
+            area_extent=(-18000000.0, -9000000.0, 18000000.0, 9000000.0))
+        with pytest.raises(NotImplementedError):
+            area_def.get_area_slices(area_to_cover)
+
 
 class TestBoundary:
     """Test 'boundary' method for AreaDefinition classes."""