Как преобразовать код с интерфейсом на Python для использования с .ui файлом (Qt Designer)?

Данный код можно адаптировать следующим образом:

• встраиваем виджет FaderTabWidget в форму Qt Designer. Подробно как это делается уже публиковалось.

Получаем fading.ui

<?xml version="1.0" encoding="UTF-8"?>
<ui version="4.0">
 <class>MainWindow</class>
 <widget class="QMainWindow" name="MainWindow">
  <property name="geometry">
   <rect>
    <x>0</x>
    <y>0</y>
    <width>691</width>
    <height>488</height>
   </rect>
  </property>
  <property name="windowTitle">
   <string>MainWindow</string>
  </property>
  <widget class="QWidget" name="centralwidget">
   <layout class="QHBoxLayout" name="horizontalLayout">
    <item>
     <widget class="FaderTabWidget" name="tabWidget">
      <property name="currentIndex">
       <number>1</number>
      </property>
      <widget class="QWidget" name="tab">
       <attribute name="title">
        <string>Tab 1</string>
       </attribute>
       <layout class="QHBoxLayout" name="horizontalLayout_2">
        <item>
         <widget class="QCalendarWidget" name="calendarWidget"/>
        </item>
       </layout>
      </widget>
      <widget class="QWidget" name="tab_2">
       <attribute name="title">
        <string>Tab 2</string>
       </attribute>
       <layout class="QHBoxLayout" name="horizontalLayout_3">
        <item>
         <widget class="QPlainTextEdit" name="plainTextEdit">
          <property name="plainText">
           <string>Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! </string>
          </property>
         </widget>
        </item>
       </layout>
      </widget>
     </widget>
    </item>
   </layout>
  </widget>
  <widget class="QMenuBar" name="menubar">
   <property name="geometry">
    <rect>
     <x>0</x>
     <y>0</y>
     <width>691</width>
     <height>21</height>
    </rect>
   </property>
  </widget>
  <widget class="QStatusBar" name="statusbar"/>
 </widget>
 <customwidgets>
  <customwidget>
   <class>FaderTabWidget</class>
   <extends>QTabWidget</extends>
   <header>fadertabwidget</header>
   <container>1</container>
  </customwidget>
 </customwidgets>
 <resources/>
 <connections/>
</ui>

• конвертируем pyuic5 fading.ui -o fading_ui.py -x

fading_ui.py

from PyQt5 import QtCore, QtGui, QtWidgets


class Ui_MainWindow(object):
    def setupUi(self, MainWindow):
        MainWindow.setObjectName("MainWindow")
        MainWindow.resize(691, 488)
        self.centralwidget = QtWidgets.QWidget(MainWindow)
        self.centralwidget.setObjectName("centralwidget")
        self.horizontalLayout = QtWidgets.QHBoxLayout(self.centralwidget)
        self.horizontalLayout.setObjectName("horizontalLayout")
        self.tabWidget = FaderTabWidget(self.centralwidget)
        self.tabWidget.setObjectName("tabWidget")
        self.tab = QtWidgets.QWidget()
        self.tab.setObjectName("tab")
        self.horizontalLayout_2 = QtWidgets.QHBoxLayout(self.tab)
        self.horizontalLayout_2.setContentsMargins(0, 0, 0, 0)
        self.horizontalLayout_2.setObjectName("horizontalLayout_2")
        self.calendarWidget = QtWidgets.QCalendarWidget(self.tab)
        self.calendarWidget.setObjectName("calendarWidget")
        self.horizontalLayout_2.addWidget(self.calendarWidget)
        self.tabWidget.addTab(self.tab, "")
        self.tab_2 = QtWidgets.QWidget()
        self.tab_2.setObjectName("tab_2")
        self.horizontalLayout_3 = QtWidgets.QHBoxLayout(self.tab_2)
        self.horizontalLayout_3.setContentsMargins(0, 0, 0, 0)
        self.horizontalLayout_3.setObjectName("horizontalLayout_3")
        self.plainTextEdit = QtWidgets.QPlainTextEdit(self.tab_2)
        self.plainTextEdit.setObjectName("plainTextEdit")
        self.horizontalLayout_3.addWidget(self.plainTextEdit)
        self.tabWidget.addTab(self.tab_2, "")
        self.horizontalLayout.addWidget(self.tabWidget)
        MainWindow.setCentralWidget(self.centralwidget)
        self.menubar = QtWidgets.QMenuBar(MainWindow)
        self.menubar.setGeometry(QtCore.QRect(0, 0, 691, 21))
        self.menubar.setObjectName("menubar")
        MainWindow.setMenuBar(self.menubar)
        self.statusbar = QtWidgets.QStatusBar(MainWindow)
        self.statusbar.setObjectName("statusbar")
        MainWindow.setStatusBar(self.statusbar)

        self.retranslateUi(MainWindow)
        self.tabWidget.setCurrentIndex(1)
        QtCore.QMetaObject.connectSlotsByName(MainWindow)

    def retranslateUi(self, MainWindow):
        _translate = QtCore.QCoreApplication.translate
        MainWindow.setWindowTitle(_translate("MainWindow", "MainWindow"))
        self.tabWidget.setTabText(self.tabWidget.indexOf(self.tab), _translate("MainWindow", "Tab 1"))
        self.plainTextEdit.setPlainText(_translate("MainWindow", "Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! Hello world! "))
        self.tabWidget.setTabText(self.tabWidget.indexOf(self.tab_2), _translate("MainWindow", "Tab 2"))


from fadertabwidget import FaderTabWidget                 # !!! 

• создаем модуль fadertabwidget.py

from PyQt5.Qt import *


class FaderWidget(QWidget):
    def __init__(self, *args, **kwargs):
        QWidget.__init__(self, *args, **kwargs)
        self.pixmap_opacity = None
        self.timeline = QTimeLine(333, self)
        self.timeline.valueChanged.connect(self.animate)
        self.timeline.finished.connect(self.close)

    def start(self, old_widget, new_widget):
        self.pixmap_opacity = 1.0
        self.old_pixmap = QPixmap(new_widget.size())
        old_widget.render(self.old_pixmap)

        self.timeline.start()

        self.resize(new_widget.size())
        self.show()

    def paintEvent(self, event):
        if self.pixmap_opacity:
            QWidget.paintEvent(self, event)
            painter = QPainter(self)
            painter.setOpacity(self.pixmap_opacity)
            painter.drawPixmap(0, 0, self.old_pixmap)

    def animate(self, value):
        self.pixmap_opacity = 1.0 - value
        self.update()


class FaderTabWidget(QTabWidget):
    def __init__(self, parent=None):
        QTabWidget.__init__(self, parent)
        self.currentChanged.connect(self.onCurrentIndex)
        self.last = -1
        self.current = self.currentIndex()

    def onCurrentIndex(self, index):
        self.last = self.current
        self.current = self.currentIndex()
        if self.widget(self.last):
            self.widget(self.last).setCurrentIndex(1)
            old_widget = self.widget(self.last).widget(0)
            current_widget = self.widget(self.current).widget(0)
            fade = self.widget(self.current).widget(1)
            fade.start(old_widget, current_widget)

    def addTab(self, widget, text):
        stack = QStackedWidget(self)
        stack.addWidget(widget)
        fade = FaderWidget(self)
        fade.timeline.finished.connect(lambda: stack.setCurrentIndex(0))
        stack.addWidget(fade)
        stack.setCurrentIndex(0 if self.currentIndex() == -1 else 1)
        QTabWidget.addTab(self, stack, text)
  

• создаем модуль main.py

import sys
from PyQt5.Qt import *

from fading_ui import Ui_MainWindow                                  # +++


class Window(QMainWindow, Ui_MainWindow):                            # +++ Ui_MainWindow
    def __init__(self):
        super(Window, self).__init__()
#        uic.loadUi('fading.ui', self)                               # ---
        self.setupUi(self)                                           # +++
        
        self.tabWidget.setTabText(0, "Tab 1")                        # +
        self.tabWidget.setTabText(1, "Tab 2")                        # +

#        self.tabWidget = FaderTabWidget()                           # ---

        self.tabWidget.addTab(QCalendarWidget(), "Tab3")
        self.tabWidget.addTab(QTextEdit('Hello World'), "Tab4")


if __name__ == "__main__":
    app = QApplication(sys.argv)
    w = Window()
    w.show()
    sys.exit(app.exec_())