MAINT clarifications in ColumnTransformer._update_transformers (scikit-learn#11323)

Author: jnothman

sklearn/compose/_column_transformer.py

@@ -290,22 +290,25 @@ def get_feature_names(self):
         return feature_names
 
     def _update_fitted_transformers(self, transformers):
+        # transformers are fitted; excludes 'drop' cases
         transformers = iter(transformers)
         transformers_ = []
 
         for name, old, column in self.transformers:
             if old == 'drop':
-                trans = old
+                trans = 'drop'
             elif old == 'passthrough':
                 # FunctionTransformer is present in list of transformers,
                 # so get next transformer, but save original string
                 next(transformers)
-                trans = old
+                trans = 'passthrough'
             else:
                 trans = next(transformers)
 
             transformers_.append((name, trans, column))
 
+        # sanity check that transformers is exhausted
+        assert not list(transformers)
         self.transformers_ = transformers_
 
     def _validate_output(self, result):