control cube iteration

lib/iris/cube.py

@@ -1240,6 +1240,16 @@ def __str__(self):
     def __repr__(self):
         return "<iris 'Cube' of %s>" % self.summary(shorten=True, name_padding=1)
 
+    def __iter__(self):
+        # Emit a warning about iterating over the cube.
+        # __getitem__ makes this possible, but now deprecated as confusing.
+        warnings.warn('Cube iteration has been deprecated: '
+                      'please use Cube.slices() instead.')
+
+        # Return a simple first-index iterator, equivalent to that produced
+        # with __getitem__, if __iter__ was not defined.
+        return (self[i] for i in range(self.shape[0]))
+
     def __getitem__(self, keys):
         """
         Cube indexing (through use of square bracket notation) has been implemented at the data level. That is,

lib/iris/tests/test_cdm.py

@@ -23,6 +23,7 @@
 
 import os
 import re
+import warnings
 
 import numpy
 
@@ -503,6 +504,20 @@ def test_ellipsis(self):
         self.assertCML([self.t[(0, 2), :, Ellipsis]], ('cube_slice', '2d_to_1d_cube_multi_slice3.cml'))
 
 
+class TestIteration(TestCube2d):
+    def test_cube_iteration(self):
+        # Check that creating a cube iterator generates a warning.
+        with warnings.catch_warnings():
+            warnings.simplefilter('error')
+            with self.assertRaises(UserWarning):
+                for subcube in self.t:  # warning->error, so this *fails*
+                    pass
+        # Check we can step through the items, and their shape and number.
+        subcubes = [subcube for subcube in self.t]
+        self.assertEqual(len(subcubes), self.t.shape[0])
+        for subcube in subcubes:
+            self.assertEqual(subcube.shape, self.t.shape[1:])
+
 class Test2dSlicing(TestCube2d):
     def test_cube_slice_all_dimensions(self):
         for cube in self.t.slices(['dim1', 'dim2']):