Ulauncher is a fast, keyboard-driven application launcher for Linux (X11/Wayland) written in Python with GTK3. It provides a global hotkey-activated search window that queries multiple sources: installed applications via .desktop files, a file browser with special path prefixes, a calculator, user-defined shortcuts (scripts/URLs), and a rich extension system. Extensions run as separate subprocesses and communicate via Unix sockets using a JSON-line IPC protocol, supporting installation from git URLs or local directories. The main process manages result buffering, keyword caching, query history, and a preferences window with theme selection, hotkey configuration, and extension management. Architecturally, Ulauncher is a single-process desktop application with a clear mode-based query routing engine. Key entry points include the CLI (`ulauncher` start/show/toggle, install/uninstall/upgrade/preview extensions) and the D-Bus-activated Gtk.Application. The codebase is highly modular with isolated utility modules (EventBus, Fallible, scheduling, subprocess utilities) that demonstrate non-trivial integration of Python patterns with the GLib event loop. The extension lifecycle is robust, with per-process state machines, error categorization, and atomic disk operations for installs/updates. The repository is actively maintained (recent commits, CI/CD pipelines, performance regression tracking, and architecture documentation). Dependencies are pinned via requirements.txt and setuptools. The code exhibits low technical drift: GTK3 is stable, deprecated APIs are isolated to compatibility layers, and the project explicitly manages backward compatibility with v2 extension API. The codebase is well-structured, with significant test coverage across all modes and utilities, making it a solid reference for integrating Python with GLib/GTK and building desktop application plugin systems.