The Web Server Gateway Interface (WSGI) 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).
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:
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-agnostic interface between web servers and web applications or frameworks to promote common ground for portable web application development.