This is the `holidays` Python library, a fast, efficient tool for generating country-, province-, state-, and financial-market-specific holiday calendars on the fly. It supports over 250 ISO 3166-1 countries (with subdivisions) and 25+ financial exchanges (NYSE, NASDAQ, LSE, etc.), using a mixin-based composition pattern to combine calendar systems (Gregorian, Islamic, Hebrew, Chinese, Hindu, Buddhist, etc.) with localization in dozens of languages. The library is designed for integration into applications that need to check whether a date is a holiday, retrieve holiday names, or export calendars to iCal format. It is not a data research project; it is a well-engineered Python library with a clean public API (`holidays.US()`, `holidays.country_holidays('US')`), extensive test coverage (100% code coverage enforced), and a robust observed-holiday rule system. The codebase demonstrates high craftsmanship through its reusable mixin pattern, configurable observed rules, and a generic iCal exporter. It is actively maintained with frequent commits, CI/CD pipelines, and dependabot integration.
Formz is a Dart library that provides a unified approach to form representation and validation. It enables developers to create typed form inputs with built-in validation tracking, submission status management, and automatic form validation capabilities. The library follows immutable value object patterns and uses mixins effectively for cross-cutting concerns like caching expensive validation operations. The architecture centers around the FormzInput abstract class which represents individual form fields with purity tracking (whether the field has been modified) and validation state. Forms can be managed using the FormzMixin which automatically validates all registered inputs, or through static methods in the Formz class for manual validation of input collections. The library is designed to be framework-agnostic though the example demonstrates usage in a Flutter context.
Lion is a monorepo maintained by ING Bank that provides a comprehensive, white-label web component library built on LitElement. It focuses on accessible, performant, and extensible form controls (inputs, checkboxes, radio groups, selects, datepickers, IBAN, phone, file, etc.), overlays (dialog, tooltip, popover, drawer), navigation components (accordion, tabs, pagination, steps), and localization/internationalization utilities. The library is architected around a rich mixin system (e.g., FormControlMixin, ValidateMixin, OverlayMixin, LocalizeMixin) that allows deep customization without sacrificing encapsulation. It also includes Node.js tooling: an AST-based dependency analyzer (providence-analytics), a documentation publishing tool, and a Babel plugin for extending docs. The codebase demonstrates strong software engineering practices: extensive test suites, accessibility-first design, scoped custom element registries, and a modular export map. However, there are signs of technical drift (outdated dependencies like ESLint 8, TypeScript 4.9, Chai 4) and some duplicated code (e.g., localizeNamespaceLoader switch statements repeated across packages, translation drift). The monorepo uses changesets for versioning and has active CI/CD.