In Python's WSGI specification, a WSGI application is a //callable// that has a specific signature. The first parameter is a dictionary, usually called {{{environ}}}, that contains all sorts of information related to the current web request. This includes common {{{CGI}}} environment variables, information about the WSGI server, and any pieces of information that upstream WSGI applications have added to {{{environ}}}.

Pretty much anything can be added to environ but whatever is added only lasts from the point of its introduction into the environment until the end of the current web request.

A common trick in a WSGI stack of applications is to modify the incoming request to make it appear like a different type of request. This is especially useful in TiddlyWeb as a way of reusing existing code.