Stackless Python

With Stackless Python, a running program is split into microthreads that are managed by the language interpreter itself, not the operating system kernel—context switching and task scheduling is done purely in the interpreter (these are thus also regarded as a form of green thread). Microthreads manage the execution of different subtasks in a program on the same CPU core. Thus, they are an alternative to event-based asynchronous programming and also avoid the overhead of using separate threads for single-core programs (because no mode switching between user mode and kernel mode needs to be done, so CPU usage can be reduced).

Although microthreads make it easier to deal with running subtasks on a single core, Stackless Python does not remove CPython's global interpreter lock (GIL), nor does it use multiple threads and/or processes. So it allows only cooperative multitasking on a shared CPU and not parallelism (preemption was originally not available but is now in some form{{cite web |url=https://bitbucket.org/stackless-dev/stackless/wiki/Home |title=About Stackless |quote=a round robin scheduler is built in. It can be used to schedule tasklets either cooperatively or preemptively. |accessdate=26 August 2016 |archive-date=23 June 2020 |archive-url=https://web.archive.org/web/20200623233250/https://bitbucket.org/stackless-dev/stackless/wiki/Home |url-status=dead }}). To use multiple CPU cores, one would still need to build an interprocess communication system on top of Stackless Python processes.

Due to the considerable number of changes in the source, Stackless Python cannot be installed on a preexisting Python installation as an extension or library. It is instead a complete Python distribution in itself. The majority of Stackless's features have also been implemented in PyPy, a self-hosting Python interpreter and JIT compiler.{{cite web|url=https://pypy.readthedocs.org/en/latest/stackless.html|title=Application-level Stackless features — PyPy documentation|website=pypy.readthedocs.org}}

Although the whole Stackless is a separate distribution, its switching functionality has been successfully packaged as a CPython extension called greenlet.{{cite web|url=https://greenlet.readthedocs.org/en/latest/|title=greenlet: Lightweight concurrent programming — greenlet 0.4.0 documentation|website=greenlet.readthedocs.org}} It is used by a number of libraries (e.g. gevent{{cite web|url=https://www.gevent.org|title=What is gevent? — gevent 1.3.0.dev0 documentation|website=www.gevent.org}}) to provide a green threading solution for CPython. Python since has received a native solution for green threads: await/async.

Stackless is used extensively in the implementation of the Eve Online massively multiplayer online game as well as in IronPort's mail platform.

{{Portal|Free and open-source software}}

• Erlang (programming language)
• Limbo (programming language)
• Go (programming language)
• SCOOP (software)

{{Reflist}}

• {{Official website}}
• Stackless Python Documentation for: [https://stackless.readthedocs.io/en/3.7-slp/stackless-python.html 3.7-slp], [https://stackless.readthedocs.io/en/3.6-slp/stackless-python.html 3.6-slp], [https://stackless.readthedocs.io/en/3.5-slp/stackless-python.html 3.5-slp], [https://stackless.readthedocs.io/en/3.4-slp/stackless-python.html 3.4-slp], [https://stackless.readthedocs.io/en/2.7-slp/stackless-python.html 2.7-slp]
• {{GitHub|stackless-dev/stackless}}
• [https://kalogirou.net/2005/08/10/multithreaded-game-scripting-with-stackless-python/ Multithreaded Game Scripting with Stackless Python] by Harry Kalogirou
• [https://web.archive.org/web/20121108011701/http://zope.stackless.com/spcpaper.htm Continuations and Stackless Python] by Christian Tismer

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Category:Concurrent computing

Category:Python (programming language) implementations

Category:Software using the Python Software Foundation License