Check area slices for correct step

pyresample/geometry.py

@@ -37,7 +37,8 @@
 from pyresample._spatial_mp import Cartesian, Cartesian_MP, Proj, Proj_MP
 from pyresample.boundary import AreaDefBoundary, Boundary, SimpleBoundary
 from pyresample.utils import (proj4_str_to_dict, proj4_dict_to_str,
-                              convert_proj_floats, proj4_radius_parameters)
+                              convert_proj_floats, proj4_radius_parameters,
+                              check_slice_orientation)
 from pyresample.area_config import create_area_def
 
 try:
@@ -1961,15 +1962,11 @@ def get_area_slices(self, area_to_cover, shape_divisible_by=None):
 
             xstart = 0 if x[0] is np.ma.masked else x[0]
             ystart = 0 if y[1] is np.ma.masked else y[1]
-            if self.area_extent[0] > self.area_extent[2]:
-                xstop = self.width if x[0] is np.ma.masked else x[0] + 1
-            else:
-                xstop = self.width if x[1] is np.ma.masked else x[1] + 1
-            if self.area_extent[1] > self.area_extent[3]:
-                ystop = self.height if y[1] is np.ma.masked else y[1] + 1
-            else:
-                ystop = self.height if y[0] is np.ma.masked else y[0] + 1
-            return slice(xstart, xstop), slice(ystart, ystop)
+            xstop = self.width if x[1] is np.ma.masked else x[1] + 1
+            ystop = self.height if y[0] is np.ma.masked else y[0] + 1
+
+            return (check_slice_orientation(slice(xstart, xstop)),
+                    check_slice_orientation(slice(ystart, ystop)))
 
         if self.proj_dict.get('proj') != 'geos':
             raise NotImplementedError("Source projection must be 'geos' if "
@@ -1999,7 +1996,8 @@ def get_area_slices(self, area_to_cover, shape_divisible_by=None):
             y_slice = _make_slice_divisible(y_slice, self.height,
                                             factor=shape_divisible_by)
 
-        return (x_slice, y_slice)
+        return (check_slice_orientation(x_slice),
+                check_slice_orientation(y_slice))
 
     def crop_around(self, other_area):
         """Crop this area around `other_area`."""

pyresample/test/test_utils.py

@@ -645,3 +645,32 @@ def test_norotation_yaml(self):
         test_area = load_area(f.name, 'regionB')
         self.assertEqual(test_area.rotation, 0)
         os.remove(f.name)
+
+
+def test_check_slice_orientation():
+    """Test that slicing fix is doing what it should."""
+    from pyresample.utils import check_slice_orientation
+
+    # Forward slicing should not be changed
+    start, stop, step = 0, 10, None
+    slice_in = slice(start, stop, step)
+    res = check_slice_orientation(slice_in)
+    assert res is slice_in
+
+    # Reverse slicing should not be changed if the step is negative
+    start, stop, step = 10, 0, -1
+    slice_in = slice(start, stop, step)
+    res = check_slice_orientation(slice_in)
+    assert res is slice_in
+
+    # Reverse slicing should be fixed if step is positive
+    start, stop, step = 10, 0, 2
+    slice_in = slice(start, stop, step)
+    res = check_slice_orientation(slice_in)
+    assert res == slice(start, stop, -step)
+
+    # Reverse slicing should be fixed if step is None
+    start, stop, step = 10, 0, None
+    slice_in = slice(start, stop, step)
+    res = check_slice_orientation(slice_in)
+    assert res == slice(start, stop, -1)

pyresample/utils/__init__.py

@@ -243,3 +243,13 @@ def recursive_dict_update(d, u):
 def is_pyproj2():
     """Determine whether the current pyproj version is >= 2.0"""
     return pyproj.__version__ >= '2'
+
+
+def check_slice_orientation(sli):
+    """Check that the slice is slicing the right way."""
+    if sli.start > sli.stop:
+        if sli.step is None or sli.step > 0:
+            step = -(sli.step or 1)
+            sli = slice(sli.start, sli.stop, step)
+
+    return sli