scikit-learn · thomasjpfan · Apr 8, 2021 · Jan 29, 2020 · Sep 25, 2020 · Sep 25, 2020
diff --git a/doc/whats_new/v1.0.rst b/doc/whats_new/v1.0.rst
@@ -105,6 +105,11 @@ Changelog
   of each transformer in `output_indices_`. :pr:`18393` by
   :user:`Luca Bittarello <lbittarello>`.
 
+- |FIX| :meth:`compose.ColumnTransformer.get_feature_names` supports
+  non-string feature names returned by any of its transformers.
+  :pr:`18459` by :user:`Albert Villanova del Moral <albertvillanova>` and
+  :user:`Alonso Silva Allende <alonsosilvaallende>`.
+
 :mod:`sklearn.datasets`
 .......................
 

diff --git a/sklearn/compose/_column_transformer.py b/sklearn/compose/_column_transformer.py
@@ -376,7 +376,7 @@ def get_feature_names(self):
                 raise AttributeError("Transformer %s (type %s) does not "
                                      "provide get_feature_names."
                                      % (str(name), type(trans).__name__))
-            feature_names.extend([name + "__" + f for f in
+            feature_names.extend([f"{name}__{f}" for f in
                                   trans.get_feature_names()])
         return feature_names
 

diff --git a/sklearn/compose/tests/test_column_transformer.py b/sklearn/compose/tests/test_column_transformer.py
@@ -748,31 +748,38 @@ def test_column_transformer_cloning():
     assert hasattr(ct.transformers_[0][1], 'mean_')
 
 
-def test_column_transformer_get_feature_names():
+def test_column_transformer_get_feature_names_raises():
     X_array = np.array([[0., 1., 2.], [2., 4., 6.]]).T
     ct = ColumnTransformer([('trans', Trans(), [0, 1])])
     # raise correct error when not fitted
     with pytest.raises(NotFittedError):
         ct.get_feature_names()
     # raise correct error when no feature names are available
     ct.fit(X_array)
-    assert_raise_message(AttributeError,
-                         "Transformer trans (type Trans) does not provide "
-                         "get_feature_names", ct.get_feature_names)
+    msg = r"Transformer trans \(type Trans\) does not provide " \
+          r"get_feature_names"
+    with pytest.raises(AttributeError, match=msg):
+        ct.get_feature_names()
 
-    # working example
-    X = np.array([[{'a': 1, 'b': 2}, {'a': 3, 'b': 4}],
-                  [{'c': 5}, {'c': 6}]], dtype=object).T
+
+@pytest.mark.parametrize("X, keys", [
+    (np.array([[{'a': 1, 'b': 2}, {'a': 3, 'b': 4}],
+               [{'c': 5}, {'c': 6}]], dtype=object).T, ('a', 'b', 'c')),
+    (np.array([[{1: 1, 2: 2}, {1: 3, 2: 4}],
+               [{3: 5}, {3: 6}]], dtype=object).T, ('1', '2', '3')),
+])
+def test_column_transformer_get_feature_names(X, keys):
     ct = ColumnTransformer(
         [('col' + str(i), DictVectorizer(), i) for i in range(2)])
     ct.fit(X)
-    assert ct.get_feature_names() == ['col0__a', 'col0__b', 'col1__c']
+    assert ct.get_feature_names() == [f'col0__{key}' for key in keys[:2]] + \
+           [f'col1__{keys[2]}']
 
     # drop transformer
     ct = ColumnTransformer(
         [('col0', DictVectorizer(), 0), ('col1', 'drop', 1)])
     ct.fit(X)
-    assert ct.get_feature_names() == ['col0__a', 'col0__b']
+    assert ct.get_feature_names() == [f'col0__{key}' for key in keys[:2]]
 
     # passthrough transformer
     ct = ColumnTransformer([('trans', 'passthrough', [0, 1])])
@@ -782,7 +789,8 @@ def test_column_transformer_get_feature_names():
     ct = ColumnTransformer([('trans', DictVectorizer(), 0)],
                            remainder='passthrough')
     ct.fit(X)
-    assert ct.get_feature_names() == ['trans__a', 'trans__b', 'x1']
+    assert ct.get_feature_names() == [f'trans__{key}' for key in keys[:2]] + \
+           ['x1']
 
     ct = ColumnTransformer([('trans', 'passthrough', [1])],
                            remainder='passthrough')