Web Server Gateway Interface

The Web Server Gateway Interface (WSGI, pronounced whiskey

• 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 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.

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.^[7]

A middleware component can perform such functions as:^[7]

• Routing a request to different application objects based on the target
  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

Examples

Example application

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 function^[8] 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.^[9] 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
  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.^[10] 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)

WSGI-compatible applications and frameworks

This section may contain indiscriminate, excessive, or irrelevant examples. Please improve the article by adding more descriptive text and removing less pertinent examples. See Wikipedia's guide to writing better articles for further suggestions. (September 2018)

Numerous web frameworks support WSGI:

Currently wrappers are available for

• 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

External links

• 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