Added extra slice hook useful for downstream

changelog/937.feature.rst

@@ -0,0 +1,5 @@
+Added ``NDCube._slice_custom_state``, an overridable no-op hook called at the
+end of ``NDCube.__getitem__`` with the newly sliced cube and the sanitized
+slice item. Subclasses carrying extra state that tracks the data axes (e.g.
+per-frame WCS objects) can override it instead of overriding ``__getitem__``
+and re-deriving the normalized item themselves.

ndcube/mixins/ndslicing.py

@@ -53,4 +53,30 @@ def __getitem__(self, item):
         if meta_is_sliceable:
             sliced_cube.meta = meta.slice[item]
 
+        self._slice_custom_state(sliced_cube, item)
+
         return sliced_cube
+
+    def _slice_custom_state(self, sliced_cube, item):
+        """
+        Update custom subclass state on a newly sliced cube.
+
+        Called at the end of ``__getitem__``, after the data, WCS, coords and
+        metadata of ``sliced_cube`` have been set.
+
+        Subclasses carrying extra state that tracks the data axes (for example
+        a list of per-frame WCS objects) should override this method instead of
+        ``__getitem__``, mutating ``sliced_cube`` in place.  The default
+        implementation does nothing.
+
+        Parameters
+        ----------
+        sliced_cube : `~ndcube.NDCube`
+            The new cube produced by slicing this one.
+
+        item : `tuple`
+            The sanitized slice item: one entry per data axis of the original
+            cube, containing only `int` and `slice` objects.  Any ellipsis has
+            already been expanded and missing trailing axes filled with
+            ``slice(None)``.
+        """

ndcube/tests/test_ndcube_slice_and_crop.py

@@ -637,3 +637,28 @@ def test_crop_by_values_quantity_table_coordinate():
                                    wcs=cube.extra_coords)
     assert cropped.shape == (10, 5)
     np.testing.assert_array_equal(cropped.data, data[3:13, 1:6])
+
+
+def test_slice_custom_state_hook(ndcube_3d_ln_lt_l):
+    class StatefulCube(NDCube):
+        def _slice_custom_state(self, sliced_cube, item):
+            sliced_cube.seen_items = [*getattr(self, "seen_items", []), item]
+
+    cube = StatefulCube(ndcube_3d_ln_lt_l.data, ndcube_3d_ln_lt_l.wcs)
+
+    # The hook receives the sanitized item: one int/slice per data axis.
+    assert cube[0].seen_items == [(0, slice(None), slice(None))]
+    assert cube[..., 1:3].seen_items == [(slice(None), slice(None), slice(1, 3))]
+    assert cube[:, 1, 1:3].seen_items == [(slice(None), 1, slice(1, 3))]
+
+    # Slicing a sliced cube calls the hook again on the new cube.
+    chained = cube[..., 1:3][1:2]
+    assert chained.seen_items == [
+        (slice(None), slice(None), slice(1, 3)),
+        (slice(1, 2), slice(None), slice(None)),
+    ]
+
+
+def test_slice_custom_state_default_is_noop(ndcube_3d_ln_lt_l):
+    sliced = ndcube_3d_ln_lt_l[0]
+    assert not hasattr(sliced, "seen_items")