pyenv is a mature, widely-used CLI tool for managing multiple Python installations per-user or per-project. It operates by intercepting Python-related commands via shims (small wrapper scripts) that redirect execution to the correct Python version based on a `.python-version` file, the `PYENV_VERSION` environment variable, or a global default. The core is implemented in Bash with a plugin architecture: the `python-build` plugin handles compiling Python from source or downloading pre-built binaries (CPython, PyPy, Anaconda, Miniconda, Miniforge, GraalPy, etc.), while `pyenv-binary` (experimental) packages installed versions into relocatable archives. The codebase includes shell completions for Bash, Zsh, Fish, and PowerShell, a hook system for extensibility, and an optional C extension for fast `realpath` resolution. It is actively maintained with frequent releases, extensive CI testing across multiple OS/arch combinations, and a large community (45k+ stars). The repository is primarily an application (CLI tool) but also contains a substantial data-like collection of build definitions and patches for hundreds of Python versions.