ENH: Add preprocessing module with scaling functions (#35)

* ENH: Initialize preprocessing module

* ENH: Add minmax_scale and standardize functions

* ENH: Add apply_scaling function to select scaler by name

* TST: Add unit tests for scalers

Author: angelsevillamol

orca_python/preprocessing/__init__.py

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+"""Preprocessing module."""
+
+from .scalers import apply_scaling, minmax_scale, standardize
+
+__all__ = ["apply_scaling", "minmax_scale", "standardize"]

orca_python/preprocessing/scalers.py

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+"""Data scaling functions."""
+
+from scipy import sparse
+from sklearn.preprocessing import MinMaxScaler, StandardScaler
+from sklearn.utils.validation import check_array
+
+
+def _validate_and_align(X_train, X_test):
+    """Validate arrays as numeric 2D matrices and ensure matching feature counts.
+
+    Parameters
+    ----------
+    X_train : array-like of shape (n_samples, n_features)
+        Training feature matrix used for validation reference.
+
+    X_test : array-like of shape (m_samples, n_features), optional
+        Test feature matrix to validate against training matrix.
+
+    Returns
+    -------
+    (X_train_valid, X_test_valid) : tuple
+        Validated arrays. X_test_valid is None if X_test is None.
+
+    Raises
+    ------
+    ValueError
+        If X_test has different number of features than X_train.
+
+    """
+    X_train = check_array(X_train, accept_sparse=True, dtype="numeric")
+    if X_test is not None:
+        X_test = check_array(X_test, accept_sparse=True, dtype="numeric")
+        if X_test.shape[1] != X_train.shape[1]:
+            raise ValueError(
+                f"X_test has {X_test.shape[1]} features but X_train has {X_train.shape[1]}."
+            )
+    return X_train, X_test
+
+
+def apply_scaling(X_train, X_test=None, method=None, return_transformer=False):
+    """Apply normalization or standardization to the input data.
+
+    The preprocessing is fit on the training data and then applied to both
+    training and test data (if provided).
+
+    Parameters
+    ----------
+    X_train : array-like of shape (n_samples, n_features)
+        Feature matrix used specifically for model training.
+
+    X_test : array-like of shape (m_samples, n_features), optional
+        Feature matrix used for model evaluation and prediction.
+
+    method : {"norm", "std", None}, optional
+        - "norm": Min-Max scaling to [0, 1]
+        - "std" : Standardization (mean=0, std=1)
+        - None  : No preprocessing
+
+    return_transformer : bool, default=False
+        If True, also return the fitted scaling object.
+
+    Returns
+    -------
+    (X_train_scaled, X_test_scaled) or (X_train_scaled, X_test_scaled, scaler)
+        Scaled arrays; X_test_scaled is None if X_test is None.
+        If return_transformer=True and method=None, scaler is None.
+
+    Raises
+    ------
+    ValueError
+        If an unknown scaling method is specified.
+
+    Examples
+    --------
+    >>> import numpy as np
+    >>> from preprocessing.scalers import apply_scaling
+    >>> X_train = np.array([[1.0, 2.0], [2.0, 4.0], [3.0, 6.0]])
+    >>> X_test = np.array([[2.5, 5.0]])
+    >>> X_train_scaled, X_test_scaled = apply_scaling(X_train, X_test, method="norm")
+    >>> X_train_scaled
+    array([[0. , 0. ],
+           [0.5, 0.5],
+           [1. , 1. ]])
+    >>> X_test_scaled
+    array([[0.75, 0.75]])
+    >>> X_train_scaled, X_test_scaled = apply_scaling(X_train, X_test, method="std")
+    >>> X_train_scaled.round(3)
+    array([[-1.225, -1.225],
+           [ 0.   ,  0.   ],
+           [ 1.225,  1.225]])
+    >>> X_test_scaled.round(3)
+    array([[0.612, 0.612]])
+
+    """
+    if method is None:
+        return (X_train, X_test, None) if return_transformer else (X_train, X_test)
+
+    if not isinstance(method, str):
+        raise ValueError("Scaling method must be a string or None.")
+
+    key = method.lower()
+    if key == "norm":
+        return minmax_scale(X_train, X_test, return_transformer)
+    elif key == "std":
+        return standardize(X_train, X_test, return_transformer)
+    else:
+        raise ValueError(
+            f"Unknown scaling method '{method}'. Valid options: 'norm', 'std', None."
+        )
+
+
+def minmax_scale(X_train, X_test=None, return_transformer=False):
+    """Scale features to a fixed range between 0 and 1.
+
+    Fits scaling parameters on training data and applies the same transformation
+    to both training and test sets.
+
+    Parameters
+    ----------
+    X_train : array-like of shape (n_samples, n_features)
+        Training feature matrix used to fit scaling parameters.
+
+    X_test : array-like of shape (m_samples, n_features), optional
+        Test feature matrix to transform using fitted parameters.
+
+    return_transformer : bool, default=False
+        If True, also return the fitted scaling object.
+
+    Returns
+    -------
+    (X_train_scaled, X_test_scaled) or (X_train_scaled, X_test_scaled, scaler)
+        Scaled arrays; X_test_scaled is None if X_test is None.
+
+    Raises
+    ------
+    ValueError
+        If X_test has a different number of features than X_train.
+
+    Examples
+    --------
+    >>> import numpy as np
+    >>> from preprocessing.scalers import minmax_scale
+    >>> X_train = np.array([[1.0, 2.0], [2.0, 4.0], [3.0, 6.0]])
+    >>> X_test = np.array([[2.5, 5.0]])
+    >>> X_train_scaled, X_test_scaled = minmax_scale(X_train, X_test)
+    >>> X_train_scaled
+    array([[0. , 0. ],
+           [0.5, 0.5],
+           [1. , 1. ]])
+    >>> X_test_scaled
+    array([[0.75, 0.75]])
+
+    """
+    X_train, X_test = _validate_and_align(X_train, X_test)
+
+    scaler = MinMaxScaler(feature_range=(0.0, 1.0))
+    X_train_scaled = scaler.fit_transform(X_train)
+    X_test_scaled = scaler.transform(X_test) if X_test is not None else None
+
+    if return_transformer:
+        return X_train_scaled, X_test_scaled, scaler
+    else:
+        return X_train_scaled, X_test_scaled
+
+
+def standardize(X_train, X_test=None, return_transformer=False):
+    """Standardize features to have zero mean and unit variance.
+
+    Fits scaling parameters on training data and applies the same transformation
+    to both training and test sets. For sparse matrices, centering is disabled
+    to preserve sparsity.
+
+    Parameters
+    ----------
+    X_train : array-like of shape (n_samples, n_features)
+        Feature matrix used specifically for model training.
+
+    X_test : array-like of shape (m_samples, n_features), optional
+        Test feature matrix to transform using fitted parameters.
+
+    return_transformer: bool, default=False
+        If True, also return the fitted scaling object.
+
+    Returns
+    -------
+    (X_train_scaled, X_test_scaled) or (X_train_scaled, X_test_scaled, scaler)
+        Scaled arrays; X_test_scaled is None if X_test is None.
+
+    Raises
+    ------
+    ValueError
+        If X_test has a different number of features than X_train.
+
+    Examples
+    --------
+    >>> import numpy as np
+    >>> from preprocessing.scalers import standardize
+    >>> X_train = np.array([[1.0, 2.0], [2.0, 4.0], [3.0, 6.0]])
+    >>> X_test = np.array([[2.5, 5.0]])
+    >>> X_train_scaled, X_test_scaled = standardize(X_train, X_test)
+    >>> X_train_scaled.round(3)
+    array([[-1.225, -1.225],
+           [ 0.   ,  0.   ],
+           [ 1.225,  1.225]])
+    >>> X_test_scaled.round(3)
+    array([[0.612, 0.612]])
+
+    """
+    X_train, X_test = _validate_and_align(X_train, X_test)
+
+    scaler = (
+        StandardScaler(with_mean=False)
+        if sparse.issparse(X_train)
+        else StandardScaler()
+    )
+    X_train_scaled = scaler.fit_transform(X_train)
+    X_test_scaled = scaler.transform(X_test) if X_test is not None else None
+
+    if return_transformer:
+        return X_train_scaled, X_test_scaled, scaler
+    else:
+        return X_train_scaled, X_test_scaled

orca_python/preprocessing/tests/__init__.py

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+"""Tests for the preprocessing module."""
+
+__all__ = []