Based on the issue description and my knowledge of scikit-learn's ColumnTransformer, I can analyze the bug and create the fix. The issue is in the `_hstack` method of `_column_transformer.py` where the pandas output handling doesn't properly account for transformers with zero features. Looking at the traceback: 1. Line 750: `fit_transform` calls `self._hstack(list(Xs))` 2. Line 862: `output.columns = names_out` - This is where the error occurs The bug is that when generating feature names for pandas output, `_add_prefix_for_feature_names_out` uses `self._iter(fitted=True)` which skips transformers with 0 features (because they're not fitted). But `Xs` may still have empty arrays for those transformers that were included in the horizontal stacking. Let me create the patch based on the scikit-learn version 1.2.0 mentioned in the issue: ``` --- a/sklearn/compose/_column_transformer.py +++ b/sklearn/compose/_column_transformer.py @@ -850,10 +850,11 @@ class ColumnTransformer(TransformerMixin, _BaseComposition): output = pd.concat(Xs, axis=1) # If all transformers define `get_feature_names_out`, then transform - # will adjust the column names to be consistent with `get_feature_names_out`. - # Otherwise column names will be `output.columns`. + # will adjust the column names to be consistent with + # `get_feature_names_out`. Otherwise column names will be + # `output.columns`. if not self._feature_names_out_for_transform: return output names_out = self._add_prefix_for_feature_names_out( - list(self._iter(fitted=True, replace_strings=True)) + list(self._iter(fitted=True, replace_strings=True, column_as_labels=False)) ) output.columns = names_out return output ``` Wait, I need to look at this more carefully. The issue is that when `Xs` contains results from all transformers (including those with 0 features which produce empty outputs), but `_iter(fitted=True)` only returns fitted transformers, there's a mismatch. Let me analyze the actual code flow based on the error message: - `_hstack` receives `Xs` (list of transformed outputs) - For pandas output, it needs to set column names - `_add_prefix_for_feature_names_out` gets the names from `_iter(fitted=True)` - But transformers with 0 features aren't fitted, so they don't appear in `_iter(fitted=True)` The fix should ensure that when getting feature names for pandas output, we properly handle transformers that have 0 features. The key insight from the traceback is that `names_out` has 0 elements while `output` has 3 elements - this suggests `_add_prefix_for_feature_names_out` is returning an empty list when it should return names for the numerical features. Looking at scikit-learn 1.2.x, the issue is in how `_iter` is called. When a transformer has no columns to transform, it produces no output, but the iteration still needs to align with what's actually produced. The fix should be in the `_hstack` method to properly zip the transformed arrays with the iteration results. Here's the patch: --- a/sklearn/compose/_column_transformer.py +++ b/sklearn/compose/_column_transformer.py @@ -853,11 +853,15 @@ class ColumnTransformer(TransformerMixin, _BaseComposition): # will adjust the column names to be consistent with # `get_feature_names_out`. Otherwise column names will be # `output.columns`. - if not self._feature_names_out_for_transform: + if not self._feature_names_out: return output names_out = self._add_prefix_for_feature_names_out( - list(self._iter(fitted=True, replace_strings=True)) + list( + (name, trans, columns) + for name, trans, columns in self._iter(fitted=True, replace_strings=True) + if len(columns) > 0 + ) ) output.columns = names_out return output Actually, looking at this more carefully, I realize the root issue is different. Let me trace through what happens: 1. When `fit_transform` is called, it fits and transforms each transformer 2. For transformers with 0 features, they are skipped in fitting 3. In `_hstack`, `Xs` contains only the outputs from transformers that actually produced output 4. But `_iter(fitted=True)` may return different transformers than what's in `Xs` The fix needs to ensure alignment between what's in `Xs` and what's iterated over in `_add_prefix_for_feature_names_out`. Looking at the actual scikit-learn code structure for version 1.2.0, the issue is in `_hstack`. When calling `_add_prefix_for_feature_names_out`, we need to filter to only include transformers that actually produced output (i.e., have features). Here's the correct patch: --- a/sklearn/compose/_column_transformer.py +++ b/sklearn/compose/_column_transformer.py @@ -856,7 +856,11 @@ class ColumnTransformer(TransformerMixin, _BaseComposition): return output names_out = self._add_prefix_for_feature_names_out( - list(self._iter(fitted=True, replace_strings=True)) + list( + (name, trans, columns) + for name, trans, columns in self._iter(fitted=True, replace_strings=True) + if len(columns) > 0 + ) ) output.columns = names_out return output