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{{short description|Calling convention for web servers to forward requests to web applications written in Python}}
{{redirect|WSGI|the radio station in Springfield, Tennessee|WSGI (AM)}}

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The '''Web Server Gateway Interface''' ('''WSGI''', pronounced ''whiskey''<ref>{{cite web |last1=Simionato |first1=Michele |title=An Introduction to Web Programming with WSGI |date=June 11, 2007 |url=http://www.phyast.pitt.edu/~micheles/python/europython07/talk.html}}</ref><ref>{{cite journal |last1=Edge |first1=Jake |title=Mucking about with microframeworks |journal=LWN |date=July 9, 2019 |url=https://lwn.net/Articles/792882/}}</ref> or {{Respell|WIZ|ghee}}<ref>{{Cite web|last=Goldberg|first=Kevin|date=2016-05-09|title=An Introduction to Python WSGI Servers for Performance {{!}} AppDynamics|url=https://www.appdynamics.com/blog/engineering/an-introduction-to-python-wsgi-servers-part-1/|access-date=2020-08-20|website=Application Performance Monitoring Blog {{!}} AppDynamics|language=en-US}}</ref>) is a simple [[calling convention]] for [[web server]]s to forward requests to [[web application]]s or [[web framework|framework]]s written in the [[Python (programming language)|Python programming language]]. The current version of WSGI, version 1.0.1, is specified in [[Python Enhancement Proposal]] (PEP) 3333.<ref name=":0">{{cite web|url=https://www.python.org/dev/peps/pep-3333/|title=PEP 3333 - Python Web Server Gateway Interface v1.0.1|website=Python.org|access-date=2018-04-04}}</ref>

WSGI was originally specified as PEP-333 in 2003.<ref>{{Cite web|url=https://www.python.org/dev/peps/pep-0333/|title=PEP 333 -- Python Web Server Gateway Interface v1.0|website=Python.org|language=en|access-date=2018-04-04}}</ref> PEP-3333, published in 2010, updates the specification for {{nowrap|[[Python 3]]}}.

==Background==
In 2003, Python [[web framework]]s were typically written against only [[Common Gateway Interface|CGI]], [[FastCGI]], [[mod_python]], or some other custom [[Application programming interface|API]] of a specific [[web server]].<ref>{{cite web|url=https://www.python.org/cgi-bin/moinmoin/WebProgramming|title=FrontPage - Python Wiki|website=Python.org|access-date=2017-01-27}}</ref> To quote PEP 333:

<blockquote>Python currently boasts a wide variety of web application frameworks, such as Zope, Quixote, Webware, SkunkWeb, PSO, and Twisted Web -- to name just a few. This wide variety of choices can be a problem for new Python users, because generally speaking, their choice of web framework will limit their choice of usable web servers, and vice versa... By contrast, although Java has just as many web application frameworks available, Java's "servlet" API makes it possible for applications written with any Java web application framework to run in any web server that supports the servlet API.</blockquote>

WSGI was thus created as an implementation-neutral [[interface (computer science)|interface]] between web servers and web applications or frameworks to promote common ground for [[software portability|portable]] web application development.<ref name=":0" />

==Specification overview==
The WSGI has two sides:
* the [[server (computing)|server]]/gateway side. This is often running full web server software such as [[Apache HTTP Server|Apache]] or [[Nginx]], or is a lightweight application server that can communicate with a webserver, such as [https://www.saddi.com/software/flup/ flup].
* the application/framework side. This is a Python callable, supplied by the Python program or framework.
Between the server and the application, there may be one or more ''WSGI [[middleware|middleware components]]'', which implement both sides of the API, typically in Python code.

WSGI does not specify how the Python interpreter should be started, nor how the application object should be loaded or configured, and different frameworks and webservers achieve this in different ways.

== WSGI middleware ==
A WSGI middleware component is a Python callable that is itself a WSGI application, but may handle requests by delegating to other WSGI applications. These applications can themselves be WSGI middleware components.<ref name=":1">{{Cite web|url=https://www.python.org/dev/peps/pep-3333/#middleware-components-that-play-both-sides|title=PEP 3333 -- Python Web Server Gateway Interface v1.0.1|website=Python.org|language=en|access-date=2018-04-04}}</ref>

A middleware component can perform such functions as:<ref name=":1" />
* Routing a request to different application objects based on the target [[Uniform Resource Locator|URL]], after changing the [[environment variables]] accordingly.
* Allowing multiple applications or frameworks to run side-by-side in the same [[process (computing)|process]]
* [[Load balancing (computing)|Load balancing]] and remote processing, by forwarding requests and responses over a [[computer network|network]]
* Performing content post-processing, such as applying [[XSLT]] stylesheets

==Examples==
===Example application===
A WSGI-compatible "[["Hello, World!" program|Hello, World!]]" application written in [[Python (programming language)|Python]]:
<syntaxhighlight lang="python" line="1">
def application(environ, start_response):
    start_response("200 OK", [("Content-Type", "text/plain")])
    yield b"Hello, World!\n"
</syntaxhighlight>

Where:
* Line 1 defines a function<ref>i.e. "a function, method, class, or an instance with a <code>__call__</code> method"</ref> named <code>application</code>, which takes two parameters, <code>environ</code> and <code>start_response</code>. <code>environ</code> is a dictionary containing [[Common Gateway Interface#Environment variables|CGI environment variables]] as well as other request parameters and metadata under well-defined keys.<ref>{{Cite web|url=https://www.python.org/dev/peps/pep-3333/#environ-variables|title=PEP 3333 -- Python Web Server Gateway Interface v1.0.1|website=Python.org|language=en|access-date=2018-04-04}}</ref>  <code>start_response</code> is a callable itself, taking two positional parameters, <code>status</code> and <code>response_headers</code>.
* Line 2 calls <code>start_response</code>, specifying "200 OK" as the HTTP status and a "Content-Type" response header.
* Line 3 makes the function into a [[Generator (computer programming)|generator]]. The body of the response is returned as an iterable of [[byte string]]s.

===Example of calling an application===
A full example of a WSGI network server is outside the scope of this article. Below is a sketch of how one would call a WSGI application and retrieve its HTTP status line, response headers, and response body, as Python objects.<ref>{{cite web|url=https://www.youtube.com/watch?v=afnDANxsaYo |archive-url=https://ghostarchive.org/varchive/youtube/20211212/afnDANxsaYo| archive-date=2021-12-12 |url-status=live|title=Creating WSGI Middleware - Alan Christopher Thomas - Minted - PythonKC|date=2015-08-28|publisher=[[YouTube]]|access-date=2017-01-27}}{{cbignore}}</ref> Details of how to construct the <code>environ</code> dict have been omitted.
<syntaxhighlight lang="python3">
from io import BytesIO


def call_application(app, environ):
    status = None
    headers = None
    body = BytesIO()

    def start_response(rstatus, rheaders):
        nonlocal status, headers
        status, headers = rstatus, rheaders

    app_iter = app(environ, start_response)
    try:
        for data in app_iter:
            assert (
                status is not None and headers is not None
            ), "start_response() was not called"
            body.write(data)
    finally:
        if hasattr(app_iter, "close"):
            app_iter.close()
    return status, headers, body.getvalue()


environ = {...}  # "environ" dict
status, headers, body = call_application(app, environ)
</syntaxhighlight>

==WSGI-compatible applications and frameworks==
{{example farm|section|date=September 2018}}
Numerous [[web framework]]s support WSGI:
{{div col|colwidth=}}
* [https://github.com/jonashaag/bjoern bjoern]
* [[BlueBream]]
* bobo<ref name="bobo">{{cite web|url=http://bobo.digicool.com |title=プエラリアジェルの効果は? |website=Bobo.digicool.com |access-date=2017-01-27}}</ref>
* Bottle
* [[CherryPy]]
* [[Django (web framework)|Django]]<ref name="djangowsgi">{{cite web|url=http://www.djangoproject.com/weblog/2005/jul/18/local_server/ |title=Django without mod_python, and WSGI support &#124; Weblog &#124; Django |website=Djangoproject.com |date=2005-07-18 |access-date=2017-01-27}}</ref>
* [[Eventlet]]<ref>{{cite web|url=http://eventlet.net/doc/modules/wsgi.html |title=wsgi – WSGI server — Eventlet 0.20.1 documentation |website=Eventlet.net |access-date=2017-01-27}}</ref>
* [https://github.com/jamesroberts/fastwsgi FastWSGI]
* [[Flask (programming)|Flask]]
* Falcon (web framework)<ref>{{cite web|url=https://falconframework.org |title=Falcon - Bare-metal web API framework for Python |access-date=2017-10-22}}</ref>
* [[Gevent-FastCGI]]<ref name="gevent-fastcgi">{{cite web|url=https://pypi.python.org/pypi/gevent-fastcgi |title=gevent-fastcgi 1.0.2.1 : Python Package Index |website=Pypi.python.org |date=2015-12-06 |access-date=2017-01-27}}</ref>
* [[Google App Engine]]'s webapp2
* [[Gunicorn (HTTP server)|Gunicorn]]
* prestans<ref name="prestans">{{cite web|url=https://github.com/prestans/prestans/ |title=anomaly/prestans: A WSGI compliant REST micro-framework |website=GitHub.com |access-date=2017-01-27}}</ref>
* [[mod_wsgi]] for use with [[Apache (web server)|Apache]]<ref>{{cite web|url=http://code.google.com/p/modwsgi/ |title=Google Code Archive - Long-term storage for Google Code Project Hosting |website=Code.google.com |access-date=2017-01-27}}</ref>
* netius
* pycnic<ref name="pycnic">{{cite web|url=http://pycnic.nullism.com |title=Pycnic Framework |website=Pycnic.nullism.com |access-date=2017-01-27}}</ref>
* [[Python Paste|Paste]] component WebOb is specifically a WSGI extension. It was adopted by the [[Pylons project]].
* [[Pylons project|Pylons]]
* [[Pyramid (web framework)|Pyramid]]
* restlite<ref name="restlite">{{cite web|url=https://github.com/theintencity/restlite |title=theintencity/restlite: Light-weight RESTful server tools in Python |website=GitHub.com |access-date=2017-01-27}}</ref>
* [[Tornado (web server)|Tornado]]
* [[Trac]]
* [[TurboGears]]
* [[Uliweb]]<ref name="Uliweb">{{cite web|url=https://github.com/limodou/uliweb |title=limodou/uliweb: Simple and easy use python web framework |website=GitHub.com |access-date=2017-01-27}}</ref>
* [[uWSGI]]
* Waitress<ref name="waitress">{{cite web|url=https://docs.pylonsproject.org/projects/waitress/en/latest/ |title=waitress documentation |website=docs.pylonsproject.org |access-date=2018-09-26}}</ref>
* [[web.py]]<ref name="web.py">{{cite web|url=http://webpy.org/ |title=Welcome to |website=Web.py |date=2009-09-11 |access-date=2017-01-27}}</ref>
* [[web2py]]
* weblayer<ref name="weblayer">{{cite web|url=http://packages.python.org/weblayer |title=weblayer — weblayer v0.4.3 documentation |website=Packages.python.org |access-date=2017-01-27}}</ref>
* Werkzeug<ref name="Werkzeug">{{cite web|url=http://werkzeug.pocoo.org/ |title=Welcome &#124; Werkzeug (The Python WSGI Utility Library) |website=Werkzeug.pocoo.org |access-date=2017-01-27}}</ref>
* Radicale<ref name="Radicale">{{cite web|url=http://radicale.org/ |title=CalDAV and CardDAV Server - A Simple Calendar and Contact Server |website=Radicale.org |access-date=2017-01-27}}</ref>
{{div col end}}

Currently wrappers are available for [[FastCGI]], [[Common Gateway Interface|CGI]], [[SCGI]], [[Apache JServ Protocol|AJP]] (using flup), [[Twisted (software)|twisted.web]], Apache (using [[mod_wsgi]] or [[mod_python]]), [[Nginx]] (using ngx_http_uwsgi_module),<ref>{{cite web|url=http://nginx.org/en/docs/http/ngx_http_uwsgi_module.html |title=Module ngx_http_uwsgi_module |website=Nginx.org |access-date=2017-01-27}}</ref> [[Nginx#Nginx Unit|Nginx Unit]] (using the Python language module),<ref>{{cite web|url=https://unit.nginx.org/configuration/#python |title=Configuration — NGINX Unit |website=Unit.nginx.org |access-date=2023-05-04}}</ref> and [[Internet Information Services|Microsoft IIS]] (using WFastCGI,<ref>{{cite web|url=https://pytools.codeplex.com/wikipage?title=wfastcgi |title=Python Tools for Visual Studio - Documentation |website=Pytools.codeplex.com |access-date=2017-01-27}}</ref> isapi-wsgi,<ref>{{cite web|url=http://code.google.com/p/isapi-wsgi/ |title=Google Code Archive - Long-term storage for Google Code Project Hosting |website=Code.google.com |access-date=2017-01-27}}</ref> PyISAPIe,<ref>{{cite web|url=http://pyisapie.sourceforge.net/ |title=Python ISAPI Extension for IIS download &#124; SourceForge.net |website=Pyisapie.sourceforge.net |date=2012-04-24 |access-date=2017-01-27}}</ref> or an [[Active Server Pages|ASP]] gateway).

==See also==
* [[Asynchronous Server Gateway Interface| Asynchronous Server Gateway Interface (ASGI)]] – The spiritual successor to WSGI, adding support for asynchronous applications
* [[Rack (web server interface)|Rack]] – [[Ruby (programming language)|Ruby]] web server interface
* [[PSGI]] – [[Perl]] Web Server Gateway Interface
* [[SCGI]] – Simple Common Gateway Interface
* [[JSGI]] – [[JavaScript]] web server gateway interface

==References==
{{Reflist|30em}}

==External links==
* [https://www.python.org/dev/peps/pep-0333/ PEP 333 – Python Web Server Gateway Interface]
* [https://www.python.org/dev/peps/pep-3333/ PEP 3333 – Python Web Server Gateway Interface v1.0.1]
* [http://www.pythonpaste.org/ WSGI metaframework]
* [http://www.wsgi.org/ Comprehensive wiki about everything WSGI]
* [http://wsgi.tutorial.codepoint.net/ WSGI Tutorial]
* [https://docs.python.org/library/wsgiref.html Python standard library module wsgiref]
* [http://lucumr.pocoo.org/2007/5/21/getting-started-with-wsgi/ Getting Started with WSGI]
* [http://nwsgi.codeplex.com/ NWSGI] – .NET implementation of the Python WSGI specification for IronPython and IIS
* [http://pypi.python.org/pypi/gevent-fastcgi Gevent-FastCGI server implemented using gevent coroutine-based networking library]

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