This operation is performed by the “common.app_tree” Tree: for every Node of the tree, an “object” XML element is generated, with the following attributes: name, class, base (class). Each object contains an element for each Property (generated by the “write” method of Property) and then an “object” element for all its sub-widgets and/or sizers. Properties in the “sizer_properties” dictionary are treated in a different way, as well as the children of a sizer, which are sub-elements of “sizeritem” objects: see the source code for details.