For example, you can use PyQt5 python module to create applications.
The Qt Designer tool can be used to create ui type file and used it.
Let's see this example.
First, open the tool in Fedora distro:
The tool can be used to create GUI for your application.
The result is one file with the extension ui.
I create one main window and I save the file with the name: untitled.ui.
This file can be used with the PyQt5 python module:
from PyQt5 import QtWidgets, uic
import sys
app = QtWidgets.QApplication([])
# location of .ui type file create with PyQt5 designer
win = uic.loadUi("untitled.ui")
win.show()
app.exec_()