Merge pull request #533 from arokem/getitem_inputs

MRG: Improve error handling for `img.__getitem__`

Show helpful error when user tries to index the image object.

Author: matthew-brett

nibabel/spatialimages.py

@@ -460,7 +460,7 @@ def from_image(klass, img):
                      klass.header_class.from_header(img.header),
                      extra=img.extra.copy())
 
-    def __getitem__(self):
+    def __getitem__(self, idx):
         ''' No slicing or dictionary interface for images
         '''
         raise TypeError("Cannot slice image objects; consider slicing image "

nibabel/tests/test_spatialimages.py

@@ -312,6 +312,14 @@ def test_get_data(self):
         in_data_template = np.arange(24, dtype=np.int16).reshape((2, 3, 4))
         in_data = in_data_template.copy()
         img = img_klass(in_data, None)
+        # Can't slice into the image object:
+        with assert_raises(TypeError) as exception_manager:
+            img[0, 0, 0]
+        # Make sure the right message gets raised:
+        assert_equal(str(exception_manager.exception),
+                     ("Cannot slice image objects; consider slicing image "
+                      "array data with `img.dataobj[slice]` or "
+                      "`img.get_data()[slice]`"))
         assert_true(in_data is img.dataobj)
         out_data = img.get_data()
         assert_true(in_data is out_data)