bug(axes): enable 0-degree arrays

If arr[key] returns an element of an array, arr[key, ...] returns a
0-degree array.

pysindy/utils/axes.py

@@ -215,7 +215,7 @@ def __getitem__(self, key: Indexer | Sequence[Indexer], /):
             base_indexer = key
         output = super().__getitem__(base_indexer)
         if not isinstance(output, AxesArray):
-            return output  # why?
+            return output  # return an element from the array
         in_dim = self.shape
         key, adv_inds = standardize_indexer(self, key)
         bcast_nd, bcast_start_ax = _determine_adv_broadcasting(key, adv_inds)
@@ -386,17 +386,10 @@ def _expand_indexer_ellipsis(key: list[Indexer], ndim: int) -> list[Indexer]:
     """Replace ellipsis in indexers with the appropriate amount of slice(None)"""
     # [...].index errors if list contains numpy array
     ellind = [ind for ind, val in enumerate(key) if val is ...][0]
-    new_key = []
     n_new_dims = sum(ax_key is None or isinstance(ax_key, str) for ax_key in key)
     n_ellipsis_dims = ndim - (len(key) - n_new_dims - 1)
-    new_key = (
-        key[:ellind]
-        + n_ellipsis_dims
-        * [
-            slice(None),
-        ]
-        + key[ellind + 1 + n_ellipsis_dims :]
-    )
+    new_key = key[:ellind] + key[ellind + 1 :]
+    new_key = new_key[:ellind] + (n_ellipsis_dims * [slice(None)]) + new_key[ellind:]
     return new_key
 
 

test/utils/test_axes.py

@@ -174,6 +174,15 @@ def test_simple_slice():
     assert arr[0] == 1
 
 
+# @pytest.mark.skip  # TODO: make this pass
+def test_0d_indexer():
+    arr = AxesArray(np.ones(2), {"ax_coord": 0})
+    arr_out = arr[1, ...]
+    assert arr_out.ndim == 0
+    assert arr_out.axes == {}
+    assert arr_out[()] == 1
+
+
 def test_basic_indexing_modifies_axes():
     axes = {"ax_time": 0, "ax_coord": 1}
     arr = AxesArray(np.ones(4).reshape((2, 2)), axes)
@@ -428,3 +437,22 @@ def test_determine_adv_broadcasting():
     res_nd, res_start = axes._determine_adv_broadcasting(indexers, [])
     assert res_nd == 0
     assert res_start is None
+
+
+def test_replace_ellipsis():
+    key = [..., 0]
+    result = axes._expand_indexer_ellipsis(key, 2)
+    expected = [slice(None), 0]
+    assert result == expected
+
+
+def test_strip_ellipsis():
+    key = [1, ...]
+    result = axes._expand_indexer_ellipsis(key, 1)
+    expected = [1]
+    assert result == expected
+
+    key = [..., 1]
+    result = axes._expand_indexer_ellipsis(key, 1)
+    expected = [1]
+    assert result == expected