Undocumented in source.
An icon factory manages a collection of gtk.IconSet; a gtk.IconSet manages a set of variants of a particular icon (i.e. a gtk.IconSet contains variants for different sizes and widget states). Icons in an icon factory are named by a stock ID, which is a simple string identifying the icon. Each gtk.Style has a list of gtk.IconFactory derived from the current theme; those icon factories are consulted first when searching for an icon. If the theme doesn’t set a particular icon, GTK+ looks for the icon in a list of default icon factories, maintained by IconFactory.addDefault and IconFactory.removeDefault. Applications with icons should add a default icon factory with their icons, which will allow themes to override the icons for the application.