I want to change the interface of a written application. this application is written in python and GTK . I don't want to change the codes manually by myself but although  I need an interface designer so I can import this application to it and the graphically  apply my intended changes to it . I tried Glade and QTdesigner but they produce .ui file and I couldn't find a tool to convert back a .ui file to python code. plus that the don't open python files directly and didn't have import options. 
any solution will be appreciated. 
thanks
It really depends on the application.
If the application uses *.glade or *.ui files you can - depending on how well it is designed re-arrange certain elements and swap out container types.
If there are no such files, you are out of luck. Then the ui is "hard"-coded (as hard as python code can get..) and you have to modify the widget hirarchy by modifying python code yourself.
There is no such editor being able to extract a layout/ui file from code itself.

gtkinspector or formerly known as gtkparasite can modify properties of widgets on the fly but nothing that really modifies the python code of the running application. They sneak around the application code and modify the widget tree from back behind through means of the gtk module lib interface (correct me if I am wrong here, not totally sure).
load an already written GTK python codes into a GUI designer