Backport PR sunpy#869: Made error message more helpful when cropping to a single pixel

Author: Cadair

changelog/869.doc.rst

@@ -0,0 +1 @@
+Clarified an error message that cropping to single pixel is not supported only when ``keepdims=False`` (the default value).

docs/explaining_ndcube/slicing.rst

@@ -168,7 +168,7 @@ But if :code:`keepdims=True`, a valid NDCube is returned with N length-1 dimensi
   >>> my_cube_roi = my_cube.crop(lower_left, upper_right, lower_right, upper_left)
   Traceback (most recent call last):
     ...
-  ValueError: Input points causes cube to be cropped to a single pixel. This is not supported.
+  ValueError: Input points causes cube to be cropped to a single pixel. This is not supported when keepdims=False.
   >>> my_cube_roi_keep = my_cube.crop(lower_left, upper_right, lower_right, upper_left,
   ...                                 keepdims=True)
   >>> my_cube_roi_keep.shape

ndcube/utils/cube.py

@@ -125,7 +125,7 @@ def get_crop_item_from_points(points, wcs, crop_by_values, keepdims):
         Denotes whether cropping is done using high-level objects or "values",
         i.e. low-level objects.
 
-    keep_dims : `bool`
+    keepdims : `bool`
         If `False`, returned item will drop length-1 dimensions otherwise, item will keep length-1 dimensions.
 
     Returns
@@ -205,7 +205,7 @@ def get_crop_item_from_points(points, wcs, crop_by_values, keepdims):
     # If item will result in a scalar cube, raise an error as this is not currently supported.
     if result_is_scalar:
         raise ValueError("Input points causes cube to be cropped to a single pixel. "
-                         "This is not supported.")
+                         "This is not supported when keepdims=False.")
     return tuple(item)