The Web Server Gateway Interface (WSGI, pronounced whiskey or ) is a simple calling convention for web servers to forward requests to web applications or frameworks written in the Python programming language. The current version of WSGI, version 1.0.1, is specified in Python Enhancement Proposal (PEP) 3333.

WSGI was originally specified as PEP-333 in 2003. PEP-3333, published in 2010, updates the specification for .

Background

In 2003, Python web frameworks were typically written against only CGI, FastCGI, mod_python, or some other custom API of a specific web server. 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 between web servers and web applications or frameworks to promote common ground for portable web application development.

A middleware component can perform such functions as: named <code>application</code>, which takes two parameters, <code>environ</code> and <code>start_response</code>. <code>environ</code> is a dictionary containing CGI environment variables as well as other request parameters and metadata under well-defined keys. <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. The body of the response is returned as an iterable of byte strings.

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. 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

Numerous web frameworks support WSGI:

  • bjoern
  • BlueBream
  • bobo
  • Bottle
  • CherryPy
  • Django
  • Eventlet
  • FastWSGI
  • Flask
  • Falcon (web framework)
  • Gevent-FastCGI
  • Google App Engine's webapp2
  • Gunicorn
  • prestans
  • mod_wsgi for use with Apache
  • netius
  • pycnic
  • Paste component WebOb is specifically a WSGI extension. It was adopted by the Pylons project.
  • Pylons
  • Pyramid
  • restlite
  • Socketify
  • Tornado
  • Trac
  • TurboGears
  • Uliweb
  • uWSGI
  • Waitress
  • web.py
  • web2py
  • weblayer
  • Werkzeug
  • Radicale

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), Nginx Unit (using the Python language module), and Microsoft IIS (using WFastCGI, isapi-wsgi, PyISAPIe, or an ASP gateway).

See also

  • 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

References

  • PEP 333 – Python Web Server Gateway Interface
  • PEP 3333 – Python Web Server Gateway Interface v1.0.1
  • WSGI metaframework
  • Comprehensive wiki about everything WSGI
  • WSGI Tutorial
  • Python standard library module wsgiref
  • Getting Started with WSGI
  • NWSGI – .NET implementation of the Python WSGI specification for IronPython and IIS
  • Gevent-FastCGI server implemented using gevent coroutine-based networking library