Global Forecasting Walkthrough

API Usage

All individual forecasters (e.g. lasso / xgboost) have the same API. Use **kwargs to pass custom hyperparameters into the underlying regressor (e.g. sklearn's LinearRegression regressor in functime's linear_model forecaster). Forecasters with automated hyperparameter tuning (e.g. auto_lasso and auto_xgboost) follow a similar API design. View API reference for details and supported global forecasters.

functime also has the following benchmark models implemented as pure Polars queries.

Benchmark Forecasters

• naive: random walk forecaster
• snaive: seasonal naive forecaster

Quickstart

Want to go straight into code? Run through every forecasting example with the following script:

quickstart.py

import json
from timeit import default_timer

import polars as pl

from functime.cross_validation import train_test_split
from functime.forecasting import auto_linear_model, linear_model, naive, snaive
from functime.metrics import smape
from functime.preprocessing import scale
from functime.seasonality import add_fourier_terms

start_time = default_timer()

# Load data
y = pl.read_parquet(
    "https://github.com/functime-org/functime/raw/main/data/commodities.parquet"
)
entity_col, time_col = y.columns[:2]
X = y.select([entity_col, time_col]).pipe(add_fourier_terms(sp=12, K=6)).collect()

print("🎯 Target variable (y):\n", y)
print("📉 Exogenous variables (X):\n", X)

# Train-test splits
test_size = 3
freq = "1mo"
y_train, y_test = train_test_split(test_size)(y)
X_train, X_test = train_test_split(test_size)(X)

# Paralleized naive forecasts!
y_pred_naive = naive(freq="1mo")(y=y_train, fh=3)
y_pred_snaive = snaive(freq="1mo", sp=12)(y=y_train, fh=3)


# Univariate time-series fit with automated lags and hyperparameter tuning
auto_forecaster = auto_linear_model(
    freq=freq, test_size=test_size, min_lags=12, max_lags=18, n_splits=3, time_budget=3
)
auto_forecaster.fit(y=y_train)
# Predict
y_pred = auto_forecaster.predict(fh=test_size)
# Score
scores = smape(y_true=y_test, y_pred=y_pred)
print("✅ Predictions (univariate):\n", y_pred.sort(entity_col))
print("💯 Scores (univariate):\n", scores.sort("smape"))
print("💯 Scores summary (univariate):\n", scores.select("smape").describe())

# Retrieve best lags and hyperparameters
best_params = auto_forecaster.best_params
print(f"✨ Best parameters (y only):\n{json.dumps(best_params, indent=4)}")

# Multivariate
forecaster = linear_model(**best_params)
forecaster.fit(y=y_train, X=X_train)
# Predict
y_pred = forecaster.predict(fh=test_size, X=X_test)
# Score
scores_with_exog = smape(y_true=y_test, y_pred=y_pred)

print("✅ Predictions (multivariate):\n", y_pred.sort(entity_col))
print("💯 Scores (multivariate):\n", scores_with_exog.sort("smape"))
print(
    "💯 Scores summary (multivariate):\n", scores_with_exog.select("smape").describe()
)

# Check uplift from Fourier features
uplift = (
    scores_with_exog.join(scores, on=entity_col, suffix="_univar")
    .with_columns(
        uplift=pl.col("smape_univar") - pl.col("smape"),
        has_uplift=pl.col("smape_univar") - pl.col("smape") > 0,
    )
    .select([entity_col, "uplift", "has_uplift"])
)

# NOTE: Fourier features lead to uplift for ~20% of commodities
# However, at the expense of an overall mean and variance SMAPE
# (likely due to overfitting on seasonal features)

print("💯 Uplift:\n", uplift.sort("uplift", descending=True))
print("💯 Proportion with uplift:", uplift.get_column("has_uplift").mean())

# "Direct" strategy forecasting
best_params["max_horizons"] = test_size  # Override max_horizons
best_params["strategy"] = "direct"  # Override strategy
# Predict using the "functional" API
y_pred = linear_model(**best_params)(y=y_train, fh=test_size)

# "Ensemble" strategy forecasting
best_params["strategy"] = "ensemble"  # Override strategy

# Backtesting
y_preds = linear_model(**best_params).backtest(y=y_train, X=X_train)
print("✅ Backtests:", y_preds)

# Forecast with target transforms and feature transforms
forecaster = linear_model(
    freq="1mo",
    lags=24,
    target_transform=scale(),
    feature_transform=add_fourier_terms(sp=12, K=6),
)
y_pred = forecaster(y=y_train, fh=test_size)

elapsed_time = default_timer() - start_time
print(f"⏱️ Elapsed time: {elapsed_time}")

Prepare Data

Load a collection of time series, also known as panel data, into a polars.LazyFrame (recommended) or polars.DataFrame and split them into train/test subsets.

import polars as pl
from functime.cross_validation import train_test_split
from functime.metrics import mase
from functime.seasonality import add_calendar_effects


# Load data
y = pl.read_parquet("https://github.com/functime-org/functime/raw/main/data/commodities.parquet")
entity_col, time_col = y.columns[:2]
X = (
    y.select([entity_col, time_col])
    .pipe(add_calendar_effects(["month"]))
    .collect()
)

# Train-test splits
test_size = 3
freq = "1mo"
y_train, y_test = train_test_split(test_size)(y)
X_train, X_test = train_test_split(test_size)(X)

Supported Data Schemas

X: polars.LazyFrame | polars.DataFrame and y: polars.LazyFrame | polars.DataFrame must contain at least three columns. The first column must represent the entity / series_id dimension. The second column must represent the time dimension as an integer, pl.Date, or pl.Datetime series. Remaining columns are considered as features.

Fit / Predict / Score

functime forecasters expose sklearn-compatible .fit and .predict methods. functime.metrics contains a comprehensive range of scoring functions for both point and probabilistic forecasts.

Supported Forecast Metrics

from functime.forecasting import linear_model
from functime.metrics import mase

# Fit
forecaster = linear_model(lags=24, freq="1mo")
forecaster.fit(y=y_train)

# Predict
y_pred = forecaster.predict(fh=3)

# Score
scores = mase(y_true=y_test, y_pred=y_pred, y_train=y_train)

functime ❤️ currying

Every transformer and splitter are curried functions.

from functime.preprocessing import boxcox, impute

# Use df.pipe to chain operations together
X_splits: pl.LazyFrame = (
    X.pipe(boxcox(method="mle"))
    .pipe(impute(method="linear"))
)
# Call .collect to execute query
X_new = X_splits.collect()

You can also use any forecaster as a curried function to run fit-predict in a single line of code.

from functime.forecasting import linear_model

y_pred = linear_model(lags=24, freq="1mo")(
    y=y_train,
    fh=28,
    X=X_train,
    X_future=X_test
)

functime is lazy

transformers and splitters in cross_validation, feature_extraction, and preprocessing are lazy. These callables return LazyFrames, which represents a Lazy computation graph/query against the input DataFrame / LazyFrame. No computation is run until the collect() method is called on the LazyFrame.

X and y should be preprocessed lazily for optimal performance. Lazy evaluation allows polars to optimize all operations on the input DataFrame / LazyFrame at once. Lazy preprocessing in functime allows for more efficient group_by operations.

With lazy transforms, operations series-by-series (e.g. boxcox, impute, diff) are chained in parallel: group_by is only called once. By contrast, with eager transforms, operations series-by-series is called in sequence: group_by-aggregate is called per transform.

Global Forecasting

Every forecaster exposes a scikit-learn fit and predict API. The fit method takes y and X (optional). The predict method takes the forecast horizon fh: int, frequency alias freq: str, and X (optional).

Supported Frequency Aliases

• 1s (1 second)
• 1m (1 minute)
• 30m (30 minute)
• 1h (1 hour)
• 1d (1 day)
• 1w (1 week)
• 1mo (1 calendar month)
• 3mo (1 calendar quarter)
• 1y (1 calendar year)
• 1i (1 index count)

from functime.forecasting import linear_model
from functime.metrics import mase

# Fit
forecaster = linear_model(lags=24, freq="1mo")
forecaster.fit(y=y_train)

# Predict
y_pred = forecaster.predict(fh=3)

# Score
scores = mase(y_true=y_test, y_pred=y_pred, y_train=y_train)

Global vs Local Forecasting

functime only supports global forecasters. Global forecasters fit and predict a collection of time series using a single model. Local forecasters (e.g. ARIMA, ETS, Theta) fit and predict one series per model. Example collections of time series, which are also known as panel data, include:

• Sales across product in a retail store
• Churn rates across customer segments
• Sensor data across devices in a factory
• Delivery times across trucks in a logistics fleet

Global forecasters, trained on a collection of similar time series, consistently outperform local forecasters.¹ Most notably, all top 50 competitors in the M5 Forecasting Competition used a global LightGBM forecasting model.²

Save 100x in Cloud spend

Local forecasting is expensive and slow. To productionize forecasts at scale (>1,000 series), local models have no choice but distributed computing. Every fit-predict call per local model per series are executed in parallel across the distributed cluster. Running a distributed cluster, however, is a significant cost and time sink for any data team.

functime believes that the vast majority of businesses do not need distributed computing to produce high-quality forecasts. Every forecaster, transformer, splitter, and metric in functime operates globally across collections of time series. We rewrote every time series operation in polars for blazing fast multi-threaded parallelism.

If you are working at a reasonable-scale company, you most likely don't need Databricks to scale your forecasts. Use functime.

Benchmark Forecasters

Naive and seasonal naive forecasters are surprisingly hard to beat! You should always consider using the naive and seasonal naive forecasts as benchmarks. functime implements embarressingly parallel versions of the naive (random walk) and seasonal naive forecasters. These forecasters are expressed as pure Polars queries and executed in lazy streaming mode for speed and memory efficiency.

from functime.forecasting import naive, snaive

y_pred_naive = naive(freq="1mo")(y=y_train, fh=12)
# sp = seasonal periods (length of one seasonal cycle)
y_pred_snaive = snaive(freq="1mo", sp=12)(y=y_train, fh=12)

Exogenous Regressors

Every forecaster in functime supports exogenous regressors.

from functime.forecasting import linear_model

forecaster = linear_model(lags=24, fit_intercept=False, freq="1mo")
forecaster.fit(y=y_train, X=X_train)
y_pred = forecaster.predict(fh=3, X=X_test)

Naive Forecasting

It is best practice to run naive forecasts (random walk, seasonal naive) as benchmarks. These simple forecasting methods can be remarkably difficult to beat!³

from functime.forecasting import naive, snaive

# Random walk model
y_pred_naive = naive(freq="1mo")(y=y_train, fh=3)

# Seasonal naive model
y_pred_snaive = snaive(freq="1mo", sp=12)(y=y_train, fh=3)

Transformations / Preprocessing

Every forecaster has two optional parameters target_transform and feature_transform, which can take a single functime transformer (e.g. diff(order=1, fill_strategy="backward"), detrend(method="linear")) or a list of transformers!

• target_transform applies a transformation on y before fit and predict. An inverse transformation is then applied after predict to return the final forecast.
• feature_transform applies a transformation on X before fit and predict.

We recommend using target_transform and feature_transform to avoid common pitfalls such as inconsistent feature engineering and data leakage. Check out the API reference for preprocessing and feature_extraction for a list of supported transformations.

Target Transform

from functime.forecasting import linear_model
from functime.preprocessing import diff, scale, boxcox

# Apply first differences
forecaster = linear_model(freq="1mo", lags=12, target_transform=diff(order=1, fill_strategy="backward"))

# Or local standardization
forecaster = linear_model(freq="1mo", lags=12, target_transform=scale())

# Or Box-cox
forecaster = linear_model(freq="1mo", lags=12, target_transform=boxcox())

# Or chain first differences then box-cox!
forecaster = linear_model(
    freq="1mo",
    lags=12,
    target_transform=[
        diff(order=1, fill_strategy="backward"),
        boxcox()
    ]
)

Feature Transform

from functime.forecasting import linear_model
from functime.seasonality import add_fourier_terms
from functime.preprocessing import roll

# Include Fourier terms to model complex seasonality
forecaster = linear_model(
    freq="1mo",
    lags=12,
    feature_transform=add_fourier_terms(sp=12, K=3)
)

# Create moving average features on exogenous regressors!
forecaster = linear_model(
    freq="1mo",
    lags=12,
    feature_transform=roll(window_sizes=[6, 12], stats=["mean", "std"], freq="1mo")
)

# Or include both Fourier terms and lags!
forecaster = linear_model(
    freq="1mo",
    lags=12,
    feature_transform=[
        add_fourier_terms(sp=12, K=3),
        roll(window_sizes=[6, 12], stats=["mean", "std"], freq="1mo")
    ]
)

Target and Feature Transform

from functime.forecasting import linear_model

forecaster = linear_model(
    freq="1mo",
    lags=12,
    target_transform=scale(),
    feature_transform=add_fourier_terms(sp=12, K=3)
)

Forecast Strategies

functime supports three forecast strategies: recursive, direct multi-step, and a simple ensemble of both recursive and direct.

from functime.forecasting import linear_model

# Recursive (Default)
recursive_forecaster = linear_model(strategy="recursive")
y_pred_rec = recursive_model(y_train, fh)

# Direct
max_horizons = 12  # Number of direct models
direct_forecaster = = linear_model(strategy="direct",max_horizons=max_horizons, freq="1mo")
y_pred_dir = recursive_model(y_train, fh)

# Ensemble
ensemble_forecaster = linear_model(strategy="ensemble", max_horizons=max_horizons, freq="1mo")
y_pred_ens = ensemble_model(y=y_train, fh=3)

where max_horizons is the number of models specific to each forecast horizon. For example, if max_horizons = 12, then twelve forecasters are fitted in total: the 1-step ahead forecast, the 2-steps ahead forecast, the 3-steps ahead forecast, ..., and the final 12-steps ahead forecast.

Censored Forecasts

Most real-world datasets in e-commerce and logistics contain zeros in the target variable: e.g. periods with no sales. To address this problem, functime implements the censored_model forecaster, which trains a binary classifier and two forecasters. The binary classifier predicts the probability that a forecast falls above or below a certain threshold (e.g. zero). The final forecast is a weighted average of the above and below threshold forecasters.

from functime.forecasting import censored_model

# Load the M5 competition Walmart dataset
y_train = pl.read_parquet("data/m5_y_train_sample.parquet")
X_train = pl.read_parquet("data/m5_X_train_sample.parquet")

# Fit-predict given threshold = 0.0
y_pred = censored_model(lags=3, threshold=0.0, freq="1d")(
    y=y_train, X=X_train, fh=fh, X_future=X_test
)

Custom Classfier and Regressors

By default, censored_model uses sklearn's HistGradientBoostingClassifier and HistGradientBoostingRegressor. To use your own classifier and regressor, implement a function that takes X and y numpy arrays and returns a fitted sklearn-compatible classifier and regressor.

from sklearn.neural_network import MLPRegressor
from sklearn.ensemble import RandomForestClassifier

def regress(X: np.ndarray, y: np.ndarray):
    regressor = MLPRegressor()
    regressor.fit(X=X, y=y)
    return regressor


def classify(X: np.ndarray, y: np.ndarray):
    classifier = RandomForestClassifier()
    classifier.fit(X=X, y=y))
    return classifier


# Censored model with custom classifier and regressor
forecaster = censored_model(
    lags=3,
    threshold=0.0,
    freq="1d",
    classify=classify,
    regress=regress
)
y_pred = forecaster(y=y_train, X=X_train, fh=fh, X_future=X_test)

Automated Parameter Tuning

Forecasters in auto_forecasting automatically tune the number of lagged regressors and the model's hyperparameters (e.g. alpha for Lasso). Cross-validation, lags tuning, and model parameters tuning are performed simultaneously for maximum efficiency.

Optimal Lag Length

auto_{model} forecasters automatically select the optimal number of lags via cross-validation. These forecasters conduct a search over possible models within min_lags and max_lags. The best model is the model with the lowest average RMSE (root mean squared error) across splits.

from functime.forecasting import auto_linear_model

# Fit then predict
forecaster = auto_linear_model(min_lags=20, max_lags=24, freq="1mo")
forecaster.fit(y=y_train, X=X_train)
y_pred = forecaster.predict(fh=3, X=X_test)

# Fit and predict
y_pred = auto_linear_model(min_lags=20, max_lags=24, freq="1mo")(
    y=y_train,
    X=X_train,
    X_future=X_test,
    fh=3
)

Hyperparameter Tuning

auto_{model} forecasters automatically select the optimal number of lags via cross-validation. These forecasters conduct a search over possible models within min_lags and max_lags. The best model is the model with the lowest average RMSE (root mean squared error) across splits.

Sane Hyperparameter Defaults

Sane defaults are used if search_space or points_to_evaluate are left as None. functime specify default hyperparameters search spaces according to best-practices from industry, top Kaggle solutions, and research.

functime uses FLAML under the hood to conduct hyperparameter tuning.

from flaml import tune
from functime.forecasting import auto_lightgbm

# Specify search space, initial conditions, and time budget
search_space = {
    "reg_alpha": tune.loguniform(1e-08, 10.0),
    "reg_lambda": tune.loguniform(1e-08, 10.0),
    "num_leaves": tune.randint(
        2, 2**max_depth if max_depth > 0 else 2**DEFAULT_TREE_DEPTH
    ),
    "colsample_bytree": tune.uniform(0.4, 1.0),
    "subsample": tune.uniform(0.4, 1.0),
    "subsample_freq": tune.randint(1, 7),
    "min_child_samples": tune.qlograndint(5, 100, 5),
}
points_to_evaluate = [
    {
        "num_leaves": 31,
        "colsample_bytree": 1.0,
        "subsample": 1.0,
        "min_child_samples": 20,
    }
]
time_budget = 420

# Fit model
forecaster = auto_lightgbm(
    freq="1mo',
    min_lags=20,
    max_lags=24,
    time_budget=time_budget,
    search_space=search_space,
    points_to_evaluate=points_to_evaluate
)
forecaster.fit(y=y_train)

# Get best lags and model hyperparameters
best_params = forecaster.best_params

Backtesting

Every forecaster and auto_forecaster has a backtest method. functime supports both expanding_window_split and sliding_window_split for backtesting and cross-validation.

from functime.forecasting import linear_model

forecaster = linear_model(lags=24, fit_intercept=False, freq="1mo")
y_preds, y_resids = forecaster.backtest(
    y=y_train,
    X=X_train,
    test_size=6,
    step_size=1,
    n_splits=3,
    window_size=1,  # Only applicable if `strategy` equals "sliding"
    strategy="expanding",
    # Raises ValueError if drop_short=False and there are
    # entities with insufficient length for cross-validation
    drop_short=True
)

Probabilistic Forecasts

functime supports two methods for generating prediction intervals.

Quantile Regression

Supported by LightGBM, XGBoost, and Catboost forecasters and their automated equivalents.

from functime.forecasting import auto_lightgbm

# Forecasts at 10th and 90th percentile
y_pred_10 = auto_lightgbm(alpha=0.1, freq="1d")(y=y_train, fh=28)
y_pred_90 = auto_lightgbm(alpha=0.9, freq="1d")(y=y_train, fh=28)

Conformal Prediction

functime currently supports batch prediction intervals (EnbPI) from the paper Conformal prediction interval for dynamic time-series

from functime.conformal import conformalize
from functime.forecasting import linear_model

forecaster = linear_model(lags=24, fit_intercept=False, freq="1mo")
y_preds, y_resids = forecaster.backtest(y=y_train, X=X_train)
# Forecasts at 10th and 90th percentile
# Requires forecast (y_pred), backtest values (y_preds),
# and residuals from backtest (y_resid)
y_pred_quantiles = conformalize(
    y_pred=y_pred,
    y_preds=y_preds,
    y_resids=y_resids,
    alphas=[0.1, 0.9]
)

---

1. Montero-Manso, P., & Hyndman, R. J. (2021). Principles and algorithms for forecasting groups of time series: Locality and globality. International Journal of Forecasting, 37(4), 1632-1653. ↩

2. Makridakis, S., Spiliotis, E., & Assimakopoulos, V. (2022). M5 accuracy competition: Results, findings, and conclusions. International Journal of Forecasting. ↩

3. https://otexts.com/fpp3/simple-methods.html ↩