feature-engineering

Feature Engineering

References

See references/calendar-features-reference.md for the complete CalendarFeatures constructor, all supported features and encodings, the delegated (calendar_features parameter) vs. manual (exog) workflows, per-forecaster support, and gotchas.

See references/rolling-stats-reference.md for the complete RollingFeatures constructor, all 9 available statistics, feature name generation formula, window behavior, and kwargs_stats usage.

When to Use This Skill

Use this skill when the user wants to create features to improve forecasting accuracy: calendar/datetime features, cyclical / onehot / spline encoding, holiday distance features, rolling statistics, differencing, or data scaling.

Related skills

• Before: autocorrelation-and-lag-selection (use ACF/PACF to choose a candidate set of lags before adding rolling and calendar features)
• After: feature-selection (prune redundant features with select_features after engineering)
• After: hyperparameter-optimization (jointly tune the engineered configuration and the estimator hyperparameters)

Stop Conditions

Scan before writing code. Each row lists a rule, the symptom when it is broken, and the recovery. Full pitfall catalog: the troubleshooting-common-errors skill.

Overview

Calendar Features

There are two ways to add calendar features (month, day of week, hour, …). Both use the CalendarFeatures class; they differ in who builds the features.

Full constructor, all features/encodings, per-forecaster support, and gotchas: references/calendar-features-reference.md.

Supported features: 'year', 'month', 'week', 'day_of_week', 'day_of_month', 'day_of_year', 'weekend', 'hour', 'minute', 'second', 'quarter'. Encodings: 'cyclical' (default, sin/cos pair), 'onehot', 'spline', None. 'year' and 'weekend' are never encoded.

Delegated — calendar_features parameter (preferred when supported)

The forecaster builds the calendar features from the datetime index during both training and prediction. You do not build or pass a calendar exog, and you do not need to cover the forecast horizon manually.

from lightgbm import LGBMRegressor
from skforecast.preprocessing import CalendarFeatures
from skforecast.recursive import ForecasterRecursive

calendar = CalendarFeatures(
    features=['month', 'day_of_week', 'hour'],
    encoding='cyclical',                 # 'cyclical' | 'onehot' | 'spline' | None
    keep_original_columns=False,
)

forecaster = ForecasterRecursive(
    estimator=LGBMRegressor(),
    lags=24,
    calendar_features=calendar,          # requires a pandas DatetimeIndex
)
forecaster.fit(y=y_train)                # calendar features built automatically
predictions = forecaster.predict(steps=24)   # calendar features built automatically

Requires a DatetimeIndex (otherwise TypeError). Calendar features are added as predictors alongside lags, window features, and any exog.

Manual — build calendar features as exog

For forecasters without the calendar_features parameter, or for use in a Pipeline:

import pandas as pd
from skforecast.preprocessing import CalendarFeatures

data = data.asfreq('h')                  # DatetimeIndex with frequency set

calendar_transformer = CalendarFeatures(
    features=['month', 'week', 'day_of_week', 'hour'],
    encoding='cyclical',
    keep_original_columns=False,
)
exog_calendar = calendar_transformer.fit_transform(data)
# Columns: month_sin, month_cos, week_sin, week_cos, ...

# The exog passed to predict() must cover the entire forecast horizon.

The function form create_calendar_features(X=data, ...) does the same in one shot without a transformer. See the reference for max_values, spline_kwargs, and features_to_encode options.

Holiday Distance Features

calculate_distance_from_holiday computes the number of periods to the next and since the last holiday. The time unit is inferred from the index frequency (days, hours, minutes, …) when date_column=None, and is always days when a date column is used.

from skforecast.preprocessing import calculate_distance_from_holiday

# Index-based (preferred): unit follows the index frequency
holiday_dist = calculate_distance_from_holiday(
    X=data[['is_holiday']],          # boolean / 0-1 column
    holiday_column='is_holiday',
    fill_na=0,                       # value for rows before first / after last holiday
)
# Columns: time_to_holiday, time_since_holiday

# Series form: values used directly as the holiday indicator
holiday_dist = calculate_distance_from_holiday(
    X=data['is_holiday'],
    fill_na=0,
)

Combine with calendar features into a single exog:

exog = pd.concat([exog_calendar, holiday_dist], axis=1)

Rolling Features (Window Statistics)

from skforecast.preprocessing import RollingFeatures
from skforecast.recursive import ForecasterRecursive
from lightgbm import LGBMRegressor

# Single window size for all stats
rolling = RollingFeatures(
    stats=['mean', 'std', 'min', 'max'],
    window_sizes=7,                  # int applies same window to all stats
)

# Different window sizes per statistic
rolling = RollingFeatures(
    stats=['mean', 'std', 'min', 'max'],
    window_sizes=[7, 7, 14, 14],     # Must match length of stats
)

# Multiple RollingFeatures objects
rolling_short = RollingFeatures(stats=['mean', 'std'], window_sizes=7)
rolling_long = RollingFeatures(stats=['mean', 'std'], window_sizes=30)

forecaster = ForecasterRecursive(
    estimator=LGBMRegressor(),
    lags=24,
    window_features=[rolling_short, rolling_long],   # List of RollingFeatures
)

Available Rolling Statistics

Standard: 'mean', 'std', 'min', 'max', 'sum', 'median', 'ratio_min_max', 'coef_variation'

Exponential weighted: 'ewm' — requires kwargs_stats:

rolling = RollingFeatures(
    stats=['ewm'],
    window_sizes=7,
    kwargs_stats={'ewm': {'alpha': 0.3}},
)

The window_features parameter

The forecaster's window_features argument accepts any object (or list of objects) that implements the contract:

def transform_batch(y: pd.Series) -> pd.DataFrame:
    # Returns one row per training sample, indexed to align with y[max_window_size:]
    ...

RollingFeatures is the built-in implementation; user-defined classes that follow the same contract are accepted. See references/rolling-stats-reference.md for the full protocol and a custom-class example.

Effective window_size. The forecaster sets window_size = max(max_lag, max_size_window_features) (+ differentiation). A RollingFeatures(window_sizes=30) combined with lags=7 raises window_size to 30 — last_window must hold ≥ 30 values and the first 30 rows of training data are dropped. This is a frequent source of "not enough data to train" errors when the rolling window is much larger than the lags.

Lags-free configuration. lags=None, window_features=... is valid (and useful for purely smoothed-history models). At least one of lags / window_features must be non-None — passing both as None raises ValueError.

Multiple RollingFeatures instances. When passing a list, each instance is processed independently and their feature names are concatenated. Names must be unique across instances; collisions silently overwrite columns. Use the features_names= argument of RollingFeatures to disambiguate when reusing the same stat/window_size combination.

Runtime mutation. After construction, use forecaster.set_window_features(new_window_features) to swap the configuration (analogous to set_lags). This recomputes window_size, max_size_window_features, window_features_names, and the differentiator's window. Useful inside manual hyperparameter loops.

Interaction with differentiation. Window features are computed on the differenced series, not the original one. With differentiation=1, roll_mean_7 is the mean of seven consecutive changes, not seven raw values. Take this into account when interpreting feature importances.

Multi-series. In ForecasterRecursiveMultiSeries, the same RollingFeatures instance is applied independently to each series — configuration is global but the computed values are per-series (no leakage between series).

Classifier variant. For ForecasterRecursiveClassifier, use RollingFeaturesClassification (also in skforecast.preprocessing) instead of RollingFeatures.

Choosing window features

When window features help:

• Noisy series — a smoothed signal (mean, ewm) reduces variance the lags cannot.
• Capturing a longer scale than the lags without exploding column count: with hourly data and lags=24, a roll_mean_168 represents the weekly level with one feature instead of adding 144 more lags.
• Heteroscedastic or regime-changing series (energy, finance, traffic) — roll_std, coef_variation.
• When extreme behaviour drives decisions (peak demand, drawdowns) — roll_min, roll_max.

When they don't help:

• Short series — each large window discards training rows (window_size grows).
• Highly periodic, low-noise series — lags + calendar features are usually enough.
• Redundant ranges — a roll_mean_24 alongside lags=range(1, 25) adds little for a gradient boosting model that can already construct the average from splits.

Which statistic for which signal:

Window sizes:

• Tie them to actual seasonality: hourly → 24, 168; daily → 7, 30; weekly → 4, 13, 52; monthly → 12.
• Multi-scale usually wins: combine one short (reactive) and one long (trend) window — e.g. stats=['mean', 'std', 'mean', 'std'], window_sizes=[7, 7, 30, 30].
• Keep max_window_size < ~25-30% of len(y_train) to preserve enough training rows.

Practical recipe:

1. Baseline: lags only (from ACF/PACF) + calendar features if applicable. Measure via backtesting.
2. If residuals track recent level → add roll_mean_<seasonal_period>.
3. If errors scale with magnitude → add roll_std_<seasonal_period>.
4. If outliers/peaks matter → add roll_max or roll_min with the same window.
5. One addition per iteration, validated with backtesting. Drop it if MAE/RMSE doesn't improve on independent folds.
6. Prune with select_features at the end — typically 2-4 of 6-8 candidates survive.

Combinations that tend to work:

• Hourly electricity demand: lags=[1..24, 168] + RollingFeatures(['mean', 'std'], [24, 24]) + cyclical calendar.
• Daily sales with promotions: lags=[1..14] + RollingFeatures(['mean', 'median'], [7, 28]) + holiday distance + promo exog.
• Financial series: lags=[1..5] + RollingFeatures(['std', 'coef_variation'], [5, 20]) + differentiation=1 (window features compute on returns, which is usually what you want).
• Multi-series with heterogeneous scales: prefer coef_variation and ratio_min_max (scale-free), or apply per-series transformer_series.

Anti-patterns:

• Adding 6-8 statistics at once "just in case" — dilutes signal and increases overfitting risk.
• roll_sum without scaling the target — its magnitude dominates the lags and destabilises training.
• min / max with very long windows on trending series — the feature becomes near-constant or monotone.
• Duplicating information: keeping lags=range(1, 31) together with roll_mean_30 rarely helps a gradient boosting model — shrink the lag set when you add the rolling stat.

Differencing (Non-Stationary Series)

# Built-in — forecaster handles differencing and inverse transform automatically
forecaster = ForecasterRecursive(
    estimator=LGBMRegressor(),
    lags=24,
    differentiation=1,               # First-order differencing (removes linear trend)
    # differentiation=2,             # Second-order (removes quadratic trend)
)
forecaster.fit(y=y_train)
predictions = forecaster.predict(steps=10)   # Auto inverse-transformed

Categorical Exogenous Variables

All ML forecasters include a categorical_features parameter (default 'auto') that automatically detects and encodes non-numeric exogenous columns using an internal OrdinalEncoder (into float codes, since numpy arrays are used internally). Native categorical support is configured automatically for LightGBM, CatBoost, XGBoost, and HistGradientBoostingRegressor.

forecaster = ForecasterRecursive(
    estimator=LGBMRegressor(),
    lags=24,
    categorical_features='auto',     # Default — auto-detect non-numeric columns
)

categorical_features options:

• 'auto' (default): Auto-detect non-numeric columns after transformer_exog.
• list: Explicit column names to treat as categorical (including numeric columns).
• None: No internal categorical encoding.

Important: When categorical_features is not None, do not set categorical features directly on the estimator or via fit_kwargs. The forecaster manages the configuration internally and overwrites estimator-level settings.

Choosing an encoding strategy:

Combining transformer_exog and categorical_features: transformer_exog is applied before categorical_features detection. Scale numeric columns with transformer_exog while categorical_features='auto' handles the rest. Avoid applying both mechanisms to the same columns.

from sklearn.compose import make_column_transformer
from sklearn.preprocessing import StandardScaler

transformer_exog = make_column_transformer(
    (StandardScaler(), ['temp', 'hum']),
    remainder='passthrough',
    verbose_feature_names_out=False
).set_output(transform='pandas')

forecaster = ForecasterRecursive(
    estimator=LGBMRegressor(),
    lags=24,
    transformer_exog=transformer_exog,
    categorical_features='auto',     # Detects remaining non-numeric columns
)

Combining Features — Full Example

import pandas as pd
from skforecast.preprocessing import (
    CalendarFeatures,
    calculate_distance_from_holiday,
    RollingFeatures,
)
from skforecast.recursive import ForecasterRecursive
from sklearn.preprocessing import StandardScaler
from lightgbm import LGBMRegressor

# 1. Calendar features with cyclical encoding (no extra deps)
calendar_transformer = CalendarFeatures(
    features=['month', 'day_of_week', 'hour'],
    encoding='cyclical',
    keep_original_columns=False,
)
exog_calendar = calendar_transformer.fit_transform(data)

# 2. Holiday distance features (unit inferred from index frequency)
exog_holiday = calculate_distance_from_holiday(
    X=data[['is_holiday']],
    holiday_column='is_holiday',
    fill_na=0,
)

# 3. Combine with other exogenous variables
exog = pd.concat([exog_external, exog_calendar, exog_holiday], axis=1)

# 4. Rolling features + lags + differencing
rolling = RollingFeatures(stats=['mean', 'std'], window_sizes=[7, 14])

forecaster = ForecasterRecursive(
    estimator=LGBMRegressor(),
    lags=[1, 2, 3, 7, 14, 24],
    window_features=rolling,
    transformer_y=StandardScaler(),
    differentiation=1,
)
forecaster.fit(y=y_train, exog=exog.loc[y_train.index])
predictions = forecaster.predict(steps=10, exog=exog.loc[forecast_index])

Variant — delegate calendar features to the forecaster

On a forecaster that supports calendar_features, drop the manual calendar exog and let the forecaster build it at train and predict time. Only the non-calendar exog (holiday + external) is passed manually:

calendar = CalendarFeatures(
    features=['month', 'day_of_week', 'hour'],
    encoding='cyclical',
    keep_original_columns=False,
)

exog = pd.concat([exog_external, exog_holiday], axis=1)   # no calendar columns here

forecaster = ForecasterRecursive(
    estimator=LGBMRegressor(),
    lags=[1, 2, 3, 7, 14, 24],
    window_features=rolling,
    calendar_features=calendar,          # built automatically at fit/predict
    transformer_y=StandardScaler(),
    differentiation=1,
)
forecaster.fit(y=y_train, exog=exog.loc[y_train.index])
predictions = forecaster.predict(steps=10, exog=exog.loc[forecast_index])

Common Mistakes

1. Not encoding cyclical features: Using raw integers for hour/month/day_of_week loses the cyclical relationship (hour 23 appears far from hour 0). Use encoding='cyclical' (or 'onehot' / 'spline').
2. Forgetting frequency on index: CalendarFeatures (and the calendar_features parameter) requires a DatetimeIndex; calculate_distance_from_holiday (index mode) infers the time unit from the frequency. Always set data.asfreq('h') (or similar) first.
3. Not covering forecast horizon with calendar/holiday exog (manual path only): When building calendar features manually as exog, the predict() exog must include future dates covering the entire forecast horizon. With the delegated calendar_features parameter this is handled automatically.
4. Mixing the delegated and manual calendar paths for the same features: Passing calendar_features= and also adding manually-built calendar columns with the same names to exog raises ValueError: Duplicated feature names detected in X_train. Pick one path per feature.
5. Using calendar_features on an unsupported forecaster: Only ForecasterRecursive, ForecasterRecursiveMultiSeries, ForecasterDirect, and ForecasterDirectMultiVariate accept it. For ForecasterRecursiveClassifier, ForecasterRnn, ForecasterStats, ForecasterFoundation, and ForecasterEquivalentDate, build calendar features manually as exog.
6. Overriding max_values for 'week' or 'day_of_year': The defaults (53, 366) are intentional — they handle ISO week 53 and leap years correctly. Use the smaller value (52 / 365) only if you have verified your data never reaches the maximum.
7. Over-engineering features: Start with lags only, then add rolling features and calendar features incrementally. Validate each addition with backtesting.
8. Rolling window larger than lags without checking training size: The forecaster's window_size is max(max_lag, max_size_window_features) (+ differentiation). A 30-step rolling window with lags=7 drops the first 30 rows of training data and requires last_window of length ≥ 30 at predict time.
9. Colliding feature names across multiple RollingFeatures: When passing window_features=[wf1, wf2], names like roll_mean_7 produced by both instances overwrite each other. Override with features_names=[...] on at least one instance.
10. Forgetting that window features run on the differenced series: With differentiation=1, roll_mean_7 is the mean of seven changes, not seven raw values. Adjust interpretation accordingly.