Deep Forest (DF21) is a Python library implementing the Deep Forest ensemble learning algorithm, which serves as an alternative to deep neural networks for tabular data. The system provides both classification (CascadeForestClassifier) and regression (CascadeForestRegressor) capabilities through a cascade of decision tree ensembles. Each cascade layer consists of multiple estimators (Random Forest and Extra Trees) whose out-of-bag predictions serve as augmented features for subsequent layers. The architecture includes feature binning for efficiency, early stopping mechanisms, support for custom estimators/predictors, and a partial mode for memory-efficient training on large datasets. The codebase is organized into modular components with clear separation of concerns, including dedicated modules for binning, estimation, layer management, tree implementation, and I/O operations.