FIX Do not call get_feature_names for empty column selections (#19579)

thomasjpfan · web-flow · commit 15c2c72e27c6 · 2021-02-27T18:35:40.000+01:00
diff --git a/doc/whats_new/v0.24.rst b/doc/whats_new/v0.24.rst
@@ -12,6 +12,13 @@ Version 0.24.2
 Changelog
 ---------
 
+:mod:`sklearn.compose`
+......................
+
+- |Fix| :meth:`compose.ColumnTransformer.get_feature_names` does not call
+  :term:`get_feature_names` on transformers with an empty column selection.
+  :pr:`19579` by `Thomas Fan`_.
+
 :mod:`sklearn.ensemble`
 .......................
 
diff --git a/sklearn/compose/_column_transformer.py b/sklearn/compose/_column_transformer.py
@@ -351,8 +351,7 @@ def get_feature_names(self):
         check_is_fitted(self)
         feature_names = []
         for name, trans, column, _ in self._iter(fitted=True):
-            if trans == 'drop' or (
-                    hasattr(column, '__len__') and not len(column)):
+            if trans == 'drop' or _is_empty_column_selection(column):
                 continue
             if trans == 'passthrough':
                 if self._feature_names_in is not None:
diff --git a/sklearn/compose/tests/test_column_transformer.py b/sklearn/compose/tests/test_column_transformer.py
@@ -1420,3 +1420,13 @@ def test_sk_visual_block_remainder_fitted_numpy(remainder):
     assert visual_block.names == ('scale', 'remainder')
     assert visual_block.name_details == ([0, 2], [1])
     assert visual_block.estimators == (scaler, remainder)
+
+
+@pytest.mark.parametrize("selector", [[], [False, False]])
+def test_get_feature_names_empty_selection(selector):
+    """Test that get_feature_names is only called for transformers that
+    were selected. Non-regression test for #19550.
+    """
+    ct = ColumnTransformer([('ohe', OneHotEncoder(drop='first'), selector)])
+    ct.fit([[1, 2], [3, 4]])
+    assert ct.get_feature_names() == []
