Keep the order of NDCollection.aligned_axis_physical_types stable

changelog/983.bugfix.rst

@@ -0,0 +1 @@
+ ~ndcube.NDCollection.aligned_axis_physical_types now returns the physical types in the order of the collection's first member, instead of an order that changed from run to run.

ndcube/ndcollection.py

@@ -134,8 +134,10 @@ def aligned_axis_physical_types(self):
         collection_types = [np.array(cube.array_axis_physical_types,
                                      dtype=object)[np.array(self.aligned_axes[name])]
                             for name, cube in self.items()]
-        # Return physical types common to all members of collection for each axis.
-        return [tuple(set.intersection(*[set(cube_types[i]) for cube_types in collection_types]))
+        # Return physical types common to all members of collection for each axis,
+        # in the order of the first member so the result does not depend on set ordering.
+        return [tuple(physical_type for physical_type in collection_types[0][i]
+                      if all(physical_type in cube_types[i] for cube_types in collection_types[1:]))
                 for i in range(self.n_aligned_axes)]
 
     def __getitem__(self, item):

ndcube/tests/test_ndcollection.py

@@ -167,7 +167,5 @@ def test_aligned_dimensions(collection, expected_aligned_dimensions):
                       ('custom:pos.helioprojective.lat', 'custom:pos.helioprojective.lon'),
                       ('em.wl',)])])
 def test_aligned_axis_physical_types(collection, expected):
-    output = collection.aligned_axis_physical_types
-    assert len(output) == len(expected)
-    for output_axis_types, expect_axis_types in zip(output, expected):
-        assert set(output_axis_types) == set(expect_axis_types)
+    # The types keep the order of the first member, so they do not change from run to run.
+    assert collection.aligned_axis_physical_types == expected