ENH: Lazy in-place Cube transpose

docs/iris/src/whatsnew/contributions_1.10/newfeature_2016-May-20_lazy_cube_transpose.txt

@@ -0,0 +1 @@
+* The transpose method of a Cube now results in a lazy transposed view of the original rather than realising the data then transposing it.

lib/iris/cube.py

@@ -2802,14 +2802,14 @@ def transpose(self, new_order=None):
 
         """
         if new_order is None:
-            new_order = np.arange(self.data.ndim)[::-1]
-        elif len(new_order) != self.data.ndim:
+            new_order = np.arange(self.ndim)[::-1]
+        elif len(new_order) != self.ndim:
             raise ValueError('Incorrect number of dimensions.')
 
-        # The data needs to be copied, otherwise this view of the transposed
-        # data will not be contiguous. Ensure not to assign via the cube.data
-        # setter property since we are reshaping the cube payload in-place.
-        self._my_data = np.transpose(self.data, new_order).copy()
+        if self.has_lazy_data():
+            self._my_data = self.lazy_data().transpose(new_order)
+        else:
+            self._my_data = self.data.transpose(new_order)
 
         dim_mapping = {src: dest for dest, src in enumerate(new_order)}
 

lib/iris/tests/unit/cube/test_Cube.py

@@ -1410,5 +1410,22 @@ def test_fail_cell_measure_dims(self):
             cm_dims = self.cube.cell_measure_dims(a_cell_measure)
 
 
+class Test_transpose(tests.IrisTest):
+    def test_lazy_data(self):
+        data = np.arange(12).reshape(3, 4)
+        cube = Cube(biggus.NumpyArrayAdapter(data))
+        cube.transpose()
+        self.assertTrue(cube.has_lazy_data())
+        self.assertArrayEqual(data.T, cube.data)
+
+    def test_not_lazy_data(self):
+        data = np.arange(12).reshape(3, 4)
+        cube = Cube(data)
+        cube.transpose()
+        self.assertFalse(cube.has_lazy_data())
+        self.assertIs(data.base, cube.data.base)
+        self.assertArrayEqual(data.T, cube.data)
+
+
 if __name__ == '__main__':
     tests.main()