Alterlab Scikit Learn

Scikit-learn

Overview

This skill provides comprehensive guidance for machine learning tasks using scikit-learn, the industry-standard Python library for classical machine learning. Use this skill for classification, regression, clustering, dimensionality reduction, preprocessing, model evaluation, and building production-ready ML pipelines.

Installation

bashCopy code
# Install scikit-learn using uv
uv pip install scikit-learn

# Optional: Install visualization dependencies
uv pip install matplotlib seaborn

# Commonly used with
uv pip install pandas numpy

When to Use This Skill

Use the scikit-learn skill when:

• Building classification or regression models
• Performing clustering or dimensionality reduction
• Preprocessing and transforming data for machine learning
• Evaluating model performance with cross-validation
• Tuning hyperparameters with grid or random search
• Creating ML pipelines for production workflows
• Comparing different algorithms for a task
• Working with both structured (tabular) and text data
• Need interpretable, classical machine learning approaches

Does NOT Trigger

Quick Start

Minimal classification flow: stratified train_test_split → StandardScaler (fit on train only) → fit estimator → classification_report. For mixed numeric/categorical data, wrap preprocessing in a ColumnTransformer inside a Pipeline so it cross-validates without leakage.

See references/worked_examples.md for full copy-paste classification and mixed-data pipeline examples.

Core Capabilities

1. Supervised Learning

Comprehensive algorithms for classification and regression tasks.

Key algorithms:

• Linear models: Logistic Regression, Linear Regression, Ridge, Lasso, ElasticNet
• Tree-based: Decision Trees, Random Forest, Gradient Boosting
• Support Vector Machines: SVC, SVR with various kernels
• Ensemble methods: AdaBoost, Voting, Stacking
• Neural Networks: MLPClassifier, MLPRegressor
• Others: Naive Bayes, K-Nearest Neighbors

When to use:

• Classification: Predicting discrete categories (spam detection, image classification, fraud detection)
• Regression: Predicting continuous values (price prediction, demand forecasting)

See: references/supervised_learning.md for detailed algorithm documentation, parameters, and usage examples.

2. Unsupervised Learning

Discover patterns in unlabeled data through clustering and dimensionality reduction.

Clustering algorithms:

• Partition-based: K-Means, MiniBatchKMeans
• Density-based: DBSCAN, HDBSCAN, OPTICS
• Hierarchical: AgglomerativeClustering
• Probabilistic: Gaussian Mixture Models
• Others: MeanShift, SpectralClustering, BIRCH

Dimensionality reduction:

• Linear: PCA, TruncatedSVD, NMF
• Manifold learning: t-SNE, UMAP, Isomap, LLE
• Feature extraction: FastICA, LatentDirichletAllocation

When to use:

• Customer segmentation, anomaly detection, data visualization
• Reducing feature dimensions, exploratory data analysis
• Topic modeling, image compression

See: references/unsupervised_learning.md for detailed documentation.

3. Model Evaluation and Selection

Tools for robust model evaluation, cross-validation, and hyperparameter tuning.

Cross-validation strategies:

• KFold, StratifiedKFold (classification)
• TimeSeriesSplit (temporal data)
• GroupKFold (grouped samples)

Hyperparameter tuning:

• GridSearchCV (exhaustive search)
• RandomizedSearchCV (random sampling)
• HalvingGridSearchCV (successive halving)

Metrics:

• Classification: accuracy, precision, recall, F1-score, ROC AUC, confusion matrix
• Regression: MSE, RMSE, MAE, R², MAPE
• Clustering: silhouette score, Calinski-Harabasz, Davies-Bouldin

When to use:

• Comparing model performance objectively
• Finding optimal hyperparameters
• Preventing overfitting through cross-validation
• Understanding model behavior with learning curves

See: references/model_evaluation.md for comprehensive metrics and tuning strategies.

4. Data Preprocessing

Transform raw data into formats suitable for machine learning.

Scaling and normalization:

• StandardScaler (zero mean, unit variance)
• MinMaxScaler (bounded range)
• RobustScaler (robust to outliers)
• Normalizer (sample-wise normalization)

Encoding categorical variables:

• OneHotEncoder (nominal categories)
• OrdinalEncoder (ordered categories)
• LabelEncoder (encodes the target labels y, not features)
• TargetEncoder (encodes high-cardinality categorical features with cross-fitted target means)

Handling missing values:

• SimpleImputer (mean, median, most frequent)
• KNNImputer (k-nearest neighbors)
• IterativeImputer (multivariate imputation)

Feature engineering:

• PolynomialFeatures (interaction terms)
• KBinsDiscretizer (binning)
• Feature selection (RFE, SelectKBest, SelectFromModel)

When to use:

• Before training any algorithm that requires scaled features (SVM, KNN, Neural Networks)
• Converting categorical variables to numeric format
• Handling missing data systematically
• Creating non-linear features for linear models

See: references/preprocessing.md for detailed preprocessing techniques.

5. Pipelines and Composition

Build reproducible, production-ready ML workflows.

Key components:

• Pipeline: Chain transformers and estimators sequentially
• ColumnTransformer: Apply different preprocessing to different columns
• FeatureUnion: Combine multiple transformers in parallel
• TransformedTargetRegressor: Transform target variable

Benefits:

• Prevents data leakage in cross-validation
• Simplifies code and improves maintainability
• Enables joint hyperparameter tuning
• Ensures consistency between training and prediction

When to use:

• Always use Pipelines for production workflows
• When mixing numerical and categorical features (use ColumnTransformer)
• When performing cross-validation with preprocessing steps
• When hyperparameter tuning includes preprocessing parameters

See: references/pipelines_and_composition.md for comprehensive pipeline patterns.

Example Scripts

Two runnable end-to-end scripts ship with this skill:

bashCopy code
python scripts/classification_pipeline.py   # mixed-data preprocessing, model comparison, GridSearchCV, evaluation, feature importance
python scripts/clustering_analysis.py        # optimal-k search, K-Means/DBSCAN/Agglomerative/GMM comparison, PCA visualization

See references/worked_examples.md for what each script demonstrates and standalone copy-paste versions.

Reference Documentation

This skill includes comprehensive reference files for deep dives into specific topics:

Quick Reference

File: references/quick_reference.md

• Common import patterns and installation instructions
• Quick workflow templates for common tasks
• Algorithm selection cheat sheets
• Common patterns and gotchas
• Performance optimization tips

Supervised Learning

File: references/supervised_learning.md

• Linear models (regression and classification)
• Support Vector Machines
• Decision Trees and ensemble methods
• K-Nearest Neighbors, Naive Bayes, Neural Networks
• Algorithm selection guide

Unsupervised Learning

File: references/unsupervised_learning.md

• All clustering algorithms with parameters and use cases
• Dimensionality reduction techniques
• Outlier and novelty detection
• Gaussian Mixture Models
• Method selection guide

Model Evaluation

File: references/model_evaluation.md

• Cross-validation strategies
• Hyperparameter tuning methods
• Classification, regression, and clustering metrics
• Learning and validation curves
• Best practices for model selection

Preprocessing

File: references/preprocessing.md

• Feature scaling and normalization
• Encoding categorical variables
• Missing value imputation
• Feature engineering techniques
• Custom transformers

Pipelines and Composition

File: references/pipelines_and_composition.md

• Pipeline construction and usage
• ColumnTransformer for mixed data types
• FeatureUnion for parallel transformations
• Complete end-to-end examples
• Best practices

Worked Examples

File: references/worked_examples.md

• Quick-start classification and mixed-data pipeline snippets
• Step-by-step classification model workflow
• Step-by-step clustering analysis workflow
• What each bundled example script demonstrates

Best Practices & Troubleshooting

File: references/best_practices_and_troubleshooting.md

• Pipeline / leakage discipline, fit-on-train-only, stratified splitting
• Random-state reproducibility, metric selection, when to scale features
• Fixes for ConvergenceWarning, overfitting, and large-dataset memory errors

Common Workflows

• Classification model: load data → stratified train_test_split → ColumnTransformer preprocessing inside a Pipeline → GridSearchCV tuning → classification_report on the held-out test set.
• Clustering analysis: StandardScaler → silhouette sweep over k to pick optimal_k → fit KMeans → PCA(n_components=2) projection for visualization.

See references/worked_examples.md for the numbered, copy-paste version of each workflow.

Version notes (scikit-learn 1.8–1.9)

• LogisticRegression(penalty=...) is deprecated (removed in 1.10): use l1_ratio (0 = L2, 1 = L1, in between = elastic net) and C=np.inf for no penalty.
• SVC(probability=True) is deprecated (removed in 1.11): wrap the model as CalibratedClassifierCV(SVC(), ensemble=False) when probabilities are needed.
• MDS(metric=True/False) became metric_mds=; HDBSCAN warns unless copy is set explicitly.
• mean_squared_error(..., squared=False) is gone; use root_mean_squared_error.

Best Practices (essentials)

• Always preprocess inside a Pipeline (prevents leakage); fit on train only, transform test.
• Use stratify=y for classification splits; set random_state everywhere for reproducibility.
• Scale features for SVM/KNN/NN/PCA/regularized-linear/K-Means; tree models and Naive Bayes do not need it.
• Pick metrics for the problem: F1/accuracy on balanced data; precision/recall/ROC AUC/balanced accuracy on imbalanced data.

See references/best_practices_and_troubleshooting.md for code examples and fixes for ConvergenceWarning, overfitting, and large-dataset memory errors.

Additional Resources

• Official Documentation: https://scikit-learn.org/stable/
• User Guide: https://scikit-learn.org/stable/user_guide.html
• API Reference: https://scikit-learn.org/stable/api/index.html
• Examples Gallery: https://scikit-learn.org/stable/auto_examples/index.html