Requests is a widely-used Python HTTP library that provides a simple, elegant API for making HTTP/1.1 requests. It abstracts away the complexities of urllib3, offering features like session persistence, cookie handling, automatic content decoding, SSL/TLS verification, and various authentication mechanisms (Basic, Digest, OAuth via extensions). The library is designed as a set of loosely coupled modules: the public API (api.py) delegates to a Session object (sessions.py) which manages connection pooling and request defaults; the HTTPAdapter (adapters.py) bridges to urllib3; and models.py defines Request, PreparedRequest, and Response objects. The codebase is well-structured, extensively tested, and follows a feature-freeze policy, focusing on stability and compatibility rather than new features. It is not an application or data research project; it is a pure library distributed via PyPI.

Black is the uncompromising Python code formatter. It is a CLI tool that automatically reformats Python source code to a consistent style, with a focus on determinism and minimal configuration. The repository contains the core formatting engine, a HTTP server (blackd) for on-demand formatting, a GitHub Action for CI integration, and a Vim plugin. The architecture is modular: parsing uses a forked lib2to3 parser (blib2to3) for lenient syntax handling, then a visitor-based line generator produces formatted lines, with specialized transformers for strings, brackets, and comments. It supports Jupyter notebooks, line-range formatting, caching, and parallel execution. The codebase is exceptionally well-tested with hundreds of edge-case test files and a comprehensive test suite. It demonstrates high craftsmanship in areas like tree manipulation, concurrency patterns, and string normalization.

CacheControl is a Python library that adds HTTP caching to the popular `requests` library. It ports the caching algorithms from httplib2, providing a transparent caching layer for HTTP sessions. The library intercepts HTTP requests and responses, caching responses based on standard HTTP caching headers (Cache-Control, Expires, ETag) and optional heuristic rules. It supports multiple cache backends including in-memory dictionary, file-based cache (with separate body storage for memory efficiency), and Redis. The architecture is modular: a `CacheController` implements the core caching logic (freshness calculation, conditional requests, cache invalidation), while `CacheControlAdapter` integrates with requests' transport adapter system. The library also provides a CLI tool (`doesitcache`) for testing caching behavior. It is well-tested, type-annotated, and follows modern Python packaging practices.