fix bug in convert_categorical_to_continuous that did not handle scen…

…arios where there is only one continuous section

python/interpret-core/interpret/glassbox/_ebm/_utils.py

@@ -90,8 +90,8 @@ def convert_categorical_to_continuous(categories):
     non_float_idxs = [idx for idx in non_float_idxs if idx not in clusters]
     non_float_idxs.append(max(categories.values()) + 1)
 
-    if len(clusters) <= 1:
-        return np.empty(0, np.float64)
+    if len(clusters) == 0:
+        return np.empty(0, np.float64), [[0], [], non_float_idxs], np.nan, np.nan
 
     cluster_bounds = sorted((min(cluster_list), max(cluster_list))
                             for cluster_list in clusters.values())

python/interpret-core/tests/glassbox/ebm/test_ebm_utils.py

@@ -60,6 +60,27 @@ def test_make_bag_stratified():
     bag = make_bag(y, test_size=0.25, rng=1, is_stratified=True)
 
 
+def test_convert_categorical_to_continuous_none():
+    cuts, mapping, old_min, old_max = convert_categorical_to_continuous(
+        {"ABCD": 1, "EFGH": 2, "IJKL": 1}
+    )
+    assert len(cuts) == 0
+    assert mapping == [[0], [], [1, 2, 3]]
+    assert np.isnan(old_min)
+    assert np.isnan(old_max)
+
+
+
+def test_convert_categorical_to_continuous_single():
+    cuts, mapping, old_min, old_max = convert_categorical_to_continuous(
+        {"10": 1, "+10": 2, "30": 1}
+    )
+    assert len(cuts) == 0
+    assert mapping == [[0], [1, 2], [3]]
+    assert old_min == 10
+    assert old_max == 30
+
+
 def test_convert_categorical_to_continuous_easy():
     cuts, mapping, old_min, old_max = convert_categorical_to_continuous(
         {"10": 1, "20": 2, "30": 3}