Web Server Gateway Interface

In 2003, Python web frameworks were typically written against only CGI, FastCGI, mod_python, or some other custom API of a specific web server.{{cite web|url=https://www.python.org/cgi-bin/moinmoin/WebProgramming|title=FrontPage - Python Wiki|website=Python.org|access-date=2017-01-27}} To quote PEP 333:

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.

WSGI was thus created as an implementation-neutral interface between web servers and web applications or frameworks to promote common ground for portable web application development.

The WSGI has two sides:

• the server/gateway side. This is often running full web server software such as 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 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.

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.{{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}}

A middleware component can perform such functions as:

• Routing a request to different application objects based on the target URL, after changing the environment variables accordingly.
• Allowing multiple applications or frameworks to run side-by-side in the same process
• Load balancing and remote processing, by forwarding requests and responses over a network
• Performing content post-processing, such as applying XSLT stylesheets

A WSGI-compatible "Hello, World!" application written in Python:

def application(environ, start_response):

start_response("200 OK", [("Content-Type", "text/plain")])

yield b"Hello, World!\n"

Where:

• Line 1 defines a functioni.e. "a function, method, class, or an instance with a __call__ method" named application, which takes two parameters, environ and start_response. environ is a dictionary containing CGI environment variables as well as other request parameters and metadata under well-defined keys.{{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}} start_response is a callable itself, taking two positional parameters, status and response_headers.
• Line 2 calls start_response, specifying "200 OK" as the HTTP status and a "Content-Type" response header.
• Line 3 makes the function into a generator. The body of the response is returned as an iterable of byte strings.

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.{{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}} Details of how to construct the environ dict have been omitted.

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)

{{example farm|section|date=September 2018}}

Numerous web frameworks support WSGI:

{{div col|colwidth=}}

• [https://github.com/jonashaag/bjoern bjoern]
• BlueBream
• 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}}
• Bottle
• CherryPy
• Django{{cite web|url=http://www.djangoproject.com/weblog/2005/jul/18/local_server/ |title=Django without mod_python, and WSGI support | Weblog | Django |website=Djangoproject.com |date=2005-07-18 |access-date=2017-01-27}}
• Eventlet{{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}}
• [https://github.com/jamesroberts/fastwsgi FastWSGI]
• Flask
• Falcon (web framework){{cite web|url=https://falconframework.org |title=Falcon - Bare-metal web API framework for Python |access-date=2017-10-22}}
• 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}}
• Google App Engine's webapp2
• Gunicorn
• 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}}
• mod_wsgi for use with Apache{{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}}
• netius
• pycnic{{cite web|url=http://pycnic.nullism.com |title=Pycnic Framework |website=Pycnic.nullism.com |access-date=2017-01-27}}
• Paste component WebOb is specifically a WSGI extension. It was adopted by the Pylons project.
• Pylons
• Pyramid
• 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}}
• Tornado
• Trac
• TurboGears
• 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}}
• uWSGI
• Waitress{{cite web|url=https://docs.pylonsproject.org/projects/waitress/en/latest/ |title=waitress documentation |website=docs.pylonsproject.org |access-date=2018-09-26}}
• web.py{{cite web|url=http://webpy.org/ |title=Welcome to |website=Web.py |date=2009-09-11 |access-date=2017-01-27}}
• web2py
• 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}}
• Werkzeug{{cite web|url=http://werkzeug.pocoo.org/ |title=Welcome | Werkzeug (The Python WSGI Utility Library) |website=Werkzeug.pocoo.org |access-date=2017-01-27}}
• 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}}

{{div col end}}

Currently wrappers are available for FastCGI, CGI, SCGI, AJP (using flup), twisted.web, Apache (using mod_wsgi or mod_python), Nginx (using ngx_http_uwsgi_module),{{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}} Nginx Unit (using the Python language module),{{cite web|url=https://unit.nginx.org/configuration/#python |title=Configuration — NGINX Unit |website=Unit.nginx.org |access-date=2023-05-04}} and Microsoft IIS (using WFastCGI,{{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}} isapi-wsgi,{{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}} PyISAPIe,{{cite web|url=http://pyisapie.sourceforge.net/ |title=Python ISAPI Extension for IIS download | SourceForge.net |website=Pyisapie.sourceforge.net |date=2012-04-24 |access-date=2017-01-27}} or an ASP gateway).

• Asynchronous Server Gateway Interface (ASGI) – The spiritual successor to WSGI, adding support for asynchronous applications
• Rack – Ruby web server interface
• PSGI – Perl Web Server Gateway Interface
• SCGI – Simple Common Gateway Interface
• JSGI – JavaScript web server gateway interface

{{Reflist|30em}}

• [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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