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{{Short description|Alternative Python implementation}}
{{multiple|
{{primary sources|date=December 2017}}
{{refimprove|date=March 2013}}
}}
{{Infobox software
| name = Stackless Python
| logo =
| screenshot =
| caption =
| collapsible =
| author = Christian Tismer
| developer = Anselm Kruis
| released = {{Start date and age|1998}}
| latest release version = {{wikidata|property|edit|reference|P348}}
| latest release date = {{Start date and age|2020|01|22}}
| latest preview version = 3.9.0 alpha 0 
| latest preview date =
| frequently updated =
| programming language = [[C (programming language)|C]], [[Python (programming language)|Python]]
| operating system = [[Linux]], [[macOS]], [[Microsoft Windows|Windows]]
| platform =
| size =
| language =
| status =
| genre = [[Interpreter (computing)|Interpreter]]
| license = [[Python Software Foundation License]]
| website = {{URL|https://github.com/stackless-dev/stackless/wiki/|github.com/stackless-dev}}
}}

'''Stackless Python''', or '''Stackless''', was a [[Python (programming language)|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 Its Github repository has been archived since February 2025, and the project has been officially discontinued.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 (programming language)|C]] [[call stack]] for its own stack. In practice, Stackless Python uses the C stack, but the stack is cleared between function calls.<ref>Archived at [https://ghostarchive.org/varchive/youtube/20211211/pDkrkP0yf70 Ghostarchive]{{cbignore}} and the [https://web.archive.org/web/20140103062231/http://www.youtube.com/watch?v=pDkrkP0yf70 Wayback Machine]{{cbignore}}: {{cite AV media| url = https://www.youtube.com/watch?v=pDkrkP0yf70| title = The story of stackless Python | website=[[YouTube]]}}{{cbignore}}</ref> The most prominent feature of Stackless is [[microthread]]s, which avoid much of the overhead associated with usual operating system [[Thread (computing)|threads]]. In addition to Python features, Stackless also adds support for [[coroutine]]s, communication [[channel (programming)|channels]], and task [[serialization]].

==Design==
With Stackless Python, a running program is split into microthreads that are managed by the language interpreter itself, not the operating system [[kernel (operating system)|kernel]]—[[context switch]]ing and [[scheduling (computing)|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 [[parallel computing|parallelism]] (preemption was originally not available but is now in some form<ref>{{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 }}</ref>). 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 [[plug-in (computing)|extension]] or [[library (computing)|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 (compilers)|self-hosting]] Python interpreter and [[Just-in-time compilation|JIT compiler]].<ref>{{cite web|url=https://pypy.readthedocs.org/en/latest/stackless.html|title=Application-level Stackless features — PyPy documentation|website=pypy.readthedocs.org}}</ref>

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

==See also==
{{Portal|Free and open-source software}}
* [[Erlang (programming language)]]
* [[Limbo (programming language)]]
* [[Go (programming language)]]
* [[SCOOP (software)]]

==References==
{{Reflist}}

==External links==
* {{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

{{Python (programming language)}}

[[Category:Concurrent computing]]
[[Category:Python (programming language) implementations]]
[[Category:Software using the Python Software Foundation License]]

Stackless Python - Revision history

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