SLEP007: Feature names, their generation and the API

Author:Adrin Jalali
Status:Under Review
Type:Standards Track


This SLEP proposes the introduction of the feature_names_in_ attribute for all estimators, and the feature_names_out_ attribute for all transformers. We here discuss the generation of such attributes and their propagation through pipelines. Since for most estimators there are multiple ways to generate feature names, this SLEP does not intend to define how exactly feature names are generated for all of them.


scikit-learn has been making it easier to build complex workflows with the ColumnTransformer and it has been seeing widespread adoption. However, using it results in pipelines where it’s not clear what the input features to the final predictor are, even more so than before. For example, after fitting the following pipeline, users should ideally be able to inspect the features going into the final predictor:

X, y = fetch_openml("titanic", version=1, as_frame=True, return_X_y=True)

# We will train our classifier with the following features:
# Numeric Features:
# - age: float.
# - fare: float.
# Categorical Features:
# - embarked: categories encoded as strings {'C', 'S', 'Q'}.
# - sex: categories encoded as strings {'female', 'male'}.
# - pclass: ordinal integers {1, 2, 3}.

# We create the preprocessing pipelines for both numeric and categorical data.
numeric_features = ['age', 'fare']
numeric_transformer = Pipeline(steps=[
    ('imputer', SimpleImputer(strategy='median')),
    ('scaler', StandardScaler())])

categorical_features = ['embarked', 'sex', 'pclass']
categorical_transformer = Pipeline(steps=[
    ('imputer', SimpleImputer(strategy='constant', fill_value='missing')),
    ('onehot', OneHotEncoder(handle_unknown='ignore'))])

preprocessor = ColumnTransformer(
        ('num', numeric_transformer, numeric_features),
        ('cat', categorical_transformer, categorical_features)])

# Append classifier to preprocessing pipeline.
# Now we have a full prediction pipeline.
clf = Pipeline(steps=[('preprocessor', preprocessor),
                      ('classifier', LogisticRegression())])

X_train, X_test, y_train, y_test = train_test_split(X, y, test_size=0.2), y_train)

However, it’s impossible to interpret or even sanity-check the LogisticRegression instance that’s produced in the example, because the correspondence of the coefficients to the input features is basically impossible to figure out.

This proposal suggests adding two attributes to fitted estimators: feature_names_in_ and feature_names_out_, such that in the abovementioned example clf[-1].feature_names_in_ and clf[-2].feature_names_out_ will be:


Ideally the generated feature names describe how a feature is generated at each stage of a pipeline. For instance, cat__sex_female shows that the feature has been through a categorical preprocessing pipeline, was originally the column sex, and has been one hot encoded and is one if it was originally female. However, this is not always possible or desirable especially when a generated column is based on many columns, since the generated feature names will be too long, for example in PCA. As a rule of thumb, the following types of transformers may generate feature names which corresponds to the original features:

  • Leave columns unchanged, e.g. StandardScaler
  • Select a subset of columns, e.g. SelectKBest
  • create new columns where each column depends on at most one input column, e.g OneHotEncoder
  • Algorithms that create combinations of a fixed number of features, e.g. PolynomialFeatures, as opposed to all of them where there are many. Note that verbosity considerations and verbose_feature_names as explained later can apply here.

This proposal talks about how feature names are generated and not how they are propagated.


The API for input and output feature names includes a feature_names_in_ attribute for all estimators, and a feature_names_out_ attribute for any estimator with a transform method, i.e. they expose the generated feature names via the feature_names_out_ attribute.

Note that this SLEP also applies to resamplers the same way as transformers.

Input Feature Names

The input feature names are stored in a fitted estimator in a feature_names_in_ attribute, and are taken from the given input data, for instance a pandas data frame. This attribute will be None if the input provides no feature names.

Output Feature Names

A fitted estimator exposes the output feature names through the feature_names_out_ attribute. Here we discuss more in detail how these feature names are generated. Since for most estimators there are multiple ways to generate feature names, this SLEP does not intend to define how exactly feature names are generated for all of them. It is instead a guideline on how they could generally be generated.

As detailed bellow, some generated output features names are the same or a derived from the input feature names. In such cases, if no input feature names are provided, x0 to xn are assumed to be their names.

Feature Selector Transformers

This includes transformers which output a subset of the input features, w/o changing them. For example, if a SelectKBest transformer selects the first and the third features, and no names are provided, the feature_names_out_ will be [x0, x2].

Feature Generating Transformers

The simplest category of transformers in this section are the ones which generate a column based on a single given column. These would simply preserve the input feature names if a single new feature is generated, such as in StandardScaler, which would map 'age' to 'age'. If an input feature maps to multiple new features, a postfix is added, so that OneHotEncoder might map 'gender' to 'gender_female' 'gender_fluid' etc.

Transformers where each output feature depends on a fixed number of input features may generate descriptive names as well. For instance, a PolynomialTransformer on a small subset of features can generate an output feature name such as x[0] * x[2] ** 3.

And finally, the transformers where each output feature depends on many or all input features, generate feature names which has the form of name0 to namen, where name represents the transformer. For instance, a PCA transformer will output [pca0, ..., pcan], n being the number of PCA components.


Meta estimators can choose to prefix the output feature names given by the estimators they are wrapping or not.

By default, Pipeline adds no prefix, i.e its feature_names_out_ is the same as the feature_names_out_ of the last step, and None if the last step is not a transformer.

ColumnTransformer by default adds a prefix to the output feature names, indicating the name of the transformer applied to them. If a column is in the output as a part of passthrough, it won’t be prefixed since no operation has been applied on it.


Here we include some examples to demonstrate the behavior of output feature names:

100 features (no names) -> PCA(n_components=3)
feature_names_out_: [pca0, pca1, pca2]

100 features (no names) -> SelectKBest(k=3)
feature_names_out_: [x2, x17, x42]

[f1, ..., f100] -> SelectKBest(k=3)
feature_names_out_: [f2, f17, f42]

[cat0] -> OneHotEncoder()
feature_names_out_: [cat0_cat, cat0_dog, ...]

[f1, ..., f100] -> Pipeline(
feature_names_out_: [pca0, pca1, pca2]

[model, make, numeric0, ..., numeric100] ->
        [('cat', Pipeline(SimpleImputer(), OneHotEncoder()),
          ['model', 'make']),
         ('num', Pipeline(SimpleImputer(), PCA(n_components=3)),
          ['numeric0', ..., 'numeric100'])]
feature_names_out_: ['cat_model_100', 'cat_model_200', ...,
                     'cat_make_ABC', 'cat_make_XYZ', ...,
                     'num_pca0', 'num_pca1', 'num_pca2']

However, the following examples produce a somewhat redundant feature names:

[model, make, numeric0, ..., numeric100] ->
        ('ohe', OneHotEncoder(), ['model', 'make']),
        ('pca', PCA(n_components=3), ['numeric0', ..., 'numeric100'])
feature_names_out_: ['ohe_model_100', 'ohe_model_200', ...,
                     'ohe_make_ABC', 'ohe_make_XYZ', ...,
                     'pca_pca0', 'pca_pca1', 'pca_pca2']



To provide more control over feature names, we could add a boolean verbose_feature_names constructor argument to certain transformers. The default would reflect the description above, but changes would allow more verbose names in some transformers, say having StandardScaler map 'age' to 'scale(age)'.

In case of the ColumnTransformer example above verbose_feature_names could remove the estimator names, leading to shorter and less redundant names:

[model, make, numeric0, ..., numeric100] ->
        (OneHotEncoder(), ['model', 'make']),
        (PCA(n_components=3), ['numeric0', ..., 'numeric100']),
feature_names_out_: ['model_100', 'model_200', ...,
                     'make_ABC', 'make_XYZ', ...,
                     'pca0', 'pca1', 'pca2']

Alternative solutions to a boolean flag could include:

  • an integer: fine tuning the verbosity of the generated feature names.
  • a callable which would give further flexibility to the user to generate user defined feature names.

These alternatives may be discussed and implemented in the future if deemed necessary.

Backward Compatibility

All estimators should implement the feature_names_in_ and feature_names_out_ API. This is checked in check_estimator, and the transition is done with a FutureWarning for at least two versions to give time to third party developers to implement the API.