Substance Designer includes Qt For Python. Users can use its UI Manager class to create menus, dialogs, custom panels and other user interface elements for their plugins.
In this page you can find simple examples of using Qt for Python inside Substance Designer.
For more information about Qt for Python, please check the official documentation.
Creating Toolbars in the Application Window
Creating Toolbars in Graph Views
Loading User Interfaces created using Qt Designer
Note: Qt Designer is not included with Substance Designer. You can get it by installing the official Qt distribution for your operating system.