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
You can't. Glade had code generation features removed years ago, because that leads to unmaintainable code when it's patched by hand after generation, to add the program's internal logic. So either you:

use Glade to generate a ui file, and have to change the python code anyway to use it
or you'll have manually write some code to change the user interface

In either way, you'll have some code to write. If you have lots of changes in the user interface, then convert your program to use Glade ui files. It will take some time, but changes will be easier afterwards. If you only have a couple of changes to do, just do them in the code, it will be faster to do.
load an already written GTK python codes into a GUI designer