UserExtract is the WSGI application which manages the credentials extractor subsystem.

Every incoming request passes through UserExtract which in turn passes the request through one or more configured credentials extractors. These look for user information in the request and attempt to validate it. If valid the information is used to set tiddlyweb.usersign in the environment. If the extractor finds no valid information it returns False and the next extractor is tried. If no extractor finds valid information, tidddlyweb.usersign is set to GUEST.