BUG Fixes voting named_estiamtor bug (scikit-learn#15375)

Author: thomasjpfan

doc/whats_new/v0.22.rst

@@ -716,6 +716,14 @@ Changelog
   - ``optimize.newton_cg`
   - ``random.random_choice_csc``
 
+:mod:`sklearn.voting`
+.....................
+
+- |Fix| The `named_estimators_` attribute in :class:`voting.VotingClassifier`
+  and :class:`voting.VotingRegressor` now correctly maps to dropped estimators.
+  Previously, the `named_estimators_` mapping was incorrect whenever one of the
+  estimators was dropped. :pr:`15375` by `Thomas Fan`_.
+
 :mod:`sklearn.isotonic`
 ..................................
 

sklearn/ensemble/_voting.py

@@ -71,8 +71,13 @@ def fit(self, X, y, sample_weight=None):
             )
 
         self.named_estimators_ = Bunch()
-        for k, e in zip(self.estimators, self.estimators_):
-            self.named_estimators_[k[0]] = e
+
+        # Uses None or 'drop' as placeholder for dropped estimators
+        est_iter = iter(self.estimators_)
+        for name, est in self.estimators:
+            current_est = est if est in (None, 'drop') else next(est_iter)
+            self.named_estimators_[name] = current_est
+
         return self
 
 

sklearn/ensemble/tests/test_voting.py

@@ -560,3 +560,21 @@ def test_deprecate_none_transformer(Voter, BaseEstimator):
            "Use the string 'drop' instead.")
     with pytest.warns(DeprecationWarning, match=msg):
         est.fit(X, y)
+
+
+# TODO: Remove drop parametrize in 0.24 when None is removed in Voting*
+@pytest.mark.parametrize(
+    "Voter, BaseEstimator",
+    [(VotingClassifier, DecisionTreeClassifier),
+     (VotingRegressor, DecisionTreeRegressor)]
+)
+@pytest.mark.parametrize("drop", [None, 'drop'])
+def test_correct_named_estimator_with_drop(Voter, BaseEstimator, drop):
+    est = Voter(estimators=[('lr', drop),
+                            ('tree', BaseEstimator(random_state=0))])
+
+    with pytest.warns(None) as rec:
+        est.fit(X, y)
+    assert rec if drop is None else not rec
+
+    assert est.named_estimators_['lr'] == drop