Converting between negative data and wcs axes.

CHANGELOG.rst

@@ -21,4 +21,6 @@ Bug Fixes
   scalar ``.data``. [#79]
 - Fixed ``NDCube.world_axis_physical_types`` in cases where there is a
   ``missing`` WCS axis. [#80]
+- Fixed bugs in converting between negative data and WCS axis
+  numbers. [#91]
 

ndcube/tests/test_utils_cube.py

@@ -28,22 +28,42 @@
      ((0, missing_axis_none), 2),
      ((1, missing_axis_none), 1),
      ((0, missing_axis_0_2), 1),
-     ((1, missing_axis_1), 0)])
+     ((1, missing_axis_1), 0),
+     ((-1, missing_axis_0_2), 1),
+     ((-2, missing_axis_1), 2),
+     ((-1, missing_axis_none), 0)])
 def test_data_axis_to_wcs_axis(test_input, expected):
     assert utils.cube.data_axis_to_wcs_axis(*test_input) == expected
 
 
+@pytest.mark.parametrize("test_input", [(-2, missing_axis_0_2), (1, missing_axis_0_2)])
+def test_data_axis_to_wcs_axis_error(test_input):
+    with pytest.raises(IndexError):
+        utils.cube.data_axis_to_wcs_axis(*test_input)
+
+
 @pytest.mark.parametrize(
     "test_input,expected",
     [((None, missing_axis_none), None),
      ((0, missing_axis_none), 2),
      ((1, missing_axis_none), 1),
      ((1, missing_axis_0_2), 0),
-     ((0, missing_axis_1), 1)])
+     ((0, missing_axis_1), 1),
+     ((-1, missing_axis_0_2), None),
+     ((-2, missing_axis_0_2), 0),
+     ((-2, missing_axis_1), None),
+     ((-3, missing_axis_1), 1),
+     ((-1, missing_axis_none), 0)])
 def test_wcs_axis_to_data_axis(test_input, expected):
     assert utils.cube.wcs_axis_to_data_axis(*test_input) == expected
 
 
+@pytest.mark.parametrize("test_input", [(-10, missing_axis_0_2), (10, missing_axis_0_2)])
+def test_wcs_axis_to_data_axis_error(test_input):
+    with pytest.raises(IndexError):
+        utils.cube.data_axis_to_wcs_axis(*test_input)
+
+
 def test_select_order():
     lists = [['TIME', 'WAVE', 'HPLT-TAN',
               'HPLN-TAN'], ['WAVE', 'HPLT-TAN', 'UTC',

ndcube/utils/cube.py

@@ -15,6 +15,11 @@ def data_axis_to_wcs_axis(data_axis, missing_axis):
     if data_axis is None:
         result = None
     else:
+        if data_axis < 0:
+            data_axis = np.invert(missing_axis).sum() + data_axis
+        if data_axis > np.invert(missing_axis).sum()-1 or data_axis < 0:
+            raise IndexError("Data axis out of range.  Number data axes = {0}".format(
+                np.invert(missing_axis).sum()))
         result = len(missing_axis)-np.where(np.cumsum(
             [b is False for b in missing_axis][::-1]) == data_axis+1)[0][0]-1
     return result
@@ -25,6 +30,11 @@ def wcs_axis_to_data_axis(wcs_axis, missing_axis):
     if wcs_axis is None:
         result = None
     else:
+        if wcs_axis < 0:
+            wcs_axis = len(missing_axis) + wcs_axis
+        if wcs_axis > len(missing_axis)-1 or wcs_axis < 0:
+            raise IndexError("WCS axis out of range.  Number WCS axes = {0}".format(
+                len(missing_axis)))
         if missing_axis[wcs_axis]:
             result = None
         else: