Stackless Python

Stackless Python, or Stackless, is a discontinued Python programming language interpreter. Its GitHub repository has been archived since February 2025, and the project has been officially discontinued. It was so named because it avoids depending on the C call stack for its own stack. In practice, Stackless Python uses the C stack, but the stack is cleared between function calls. The most prominent feature of Stackless is microthreads, which avoid much of the overhead associated with usual operating system threads. In addition to Python features, Stackless also adds support for coroutines, communication channels, and task serialization. The MMORPG game EVE Online is one of the major users of Stackless Python, since 2003 (with Python 2.1, then 2.3, 2.5, 2.7), and the company behind it Fenris Creations (previously CCP Games) one of the main contributor to it, but started to move away from it, i.e. started migration to Python 3 in 2026 (in the meantime they open sourced the game engine, and one part is a component "designed to match that of Stackless Python as closely as possible" though it diverges also).