The Web Server Gateway Interface (WSGI) is a specification for simple and universal interface between web servers and web applications or frameworks for the Python programming language. It was originally specified in PEP 333 authored by Phillip J. Eby, and published on 7 December 2003. It has since been adopted as a standard for Python web application development. The latest version of the specification is v1.0.1, also known as PEP 3333, published on 26 September 2010.
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Background
Python web frameworks had been a problem for new Python users since the choice of web framework would limit the choice of usable web servers, and vice versa. Python applications were often designed for only one of CGI, FastCGI, mod_python, or some other custom API of a specific web server.
WSGI was created as a low-level interface between web servers and web applications or frameworks to promote common ground for portable web application development.
Specification overview
The WSGI has two sides: the "server" or "gateway" side (often a web server such as Apache or Nginx), and the "application" or "framework" side (the python script itself). To process a WSGI request, the server side executes the application and provides environment information and a callback function to the application side. The application processes the request, returning the response to the server side using the callback function it was provided.
Between the server and the application, there may be a WSGI middleware, which implements both sides of the API. The server receives a request from a client and forwards it to the middleware. After processing, it sends a request to the application. The application's response is forwarded by the middleware to the server and ultimately to the client. There may be multiple middlewares forming a stack of WSGI-compliant applications.
A "middleware" component can perform such functions as:
Example application
A WSGI-compatible “Hello World” application written in Python:
Where:
application, which takes two parameters, environ and start_response. environ is a dictionary containing environment variables in CGI. start_response is a callable taking two required parameters status and response_headers.start_response, specifying "200 OK" as the status and a "Content-Type" header.Example of calling an application
An example of calling an application and retrieving its response. A web server gives a callback start_response to a web framework application. A start_response has an http protocol status, headers, and body. For Django, an HttpResponse object is returned. The "environ" fragment consists of, for example, REQUEST_METHOD info. Depending on this info, it might or might not call ("OPTIONS") this app.
WSGI-compatible applications and frameworks
Numerous web frameworks support WSGI:
Wrappers
The server or gateway invokes the application callable once for each request it receives from an HTTP client that is directed at the application.
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), and Microsoft IIS (using WFastCGI, isapi-wsgi, PyISAPIe, or an ASP gateway).
WSGI and Python 3
The separation of binary and text data in Python 3 poses a problem for WSGI, as it specifies that header data should be strings, while it sometimes needs to be binary and sometimes text. This works in Python 2 where text and binary data both are kept in "string" variables, but in Python 3 binary data is kept in "bytes" variables and "string" variables are for unicode text data. An updated version of the WSGI specification that deals with this is PEP 3333.
A reworked WSGI spec Web3 has also been proposed, specified in PEP444. This standard is an incompatible derivative of WSGI designed to work on Python 2.6, 2.7, 3.1+.