1. mainwindow.ui

At Qt Creator, create a new Qt Design Form, choose “Main Window” for template, and save as mainwindow.ui.
Add a QPushButton to the center of the centralwidget.

Qt Design Form.png

Your file (mainwindow.ui) should look something like this:

<?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>400</width>
    <height>300</height>
   </rect>
  </property>
  <property name="windowTitle">
   <string>MainWindow</string>
  </property>
  <widget class="QWidget" name="centralWidget">
   <widget class="QPushButton" name="pushButton">
    <property name="geometry">
     <rect>
      <x>110</x>
      <y>80</y>
      <width>201</width>
      <height>81</height>
     </rect>
    </property>
    <property name="text">
     <string>PushButton</string>
    </property>
   </widget>
  </widget>
  <widget class="QMenuBar" name="menuBar">
   <property name="geometry">
    <rect>
     <x>0</x>
     <y>0</y>
     <width>400</width>
     <height>20</height>
    </rect>
   </property>
  </widget>
  <widget class="QToolBar" name="mainToolBar">
   <attribute name="toolBarArea">
    <enum>TopToolBarArea</enum>
   </attribute>
   <attribute name="toolBarBreak">
    <bool>false</bool>
   </attribute>
  </widget>
  <widget class="QStatusBar" name="statusBar"/>
 </widget>
 <layoutdefault spacing="6" margin="11"/>
 <resources/>
 <connections/>
</ui>

生成一个 Python类

from ui_mainwindow import Ui_MainWindow
使用 pyside2-uic 来生成与 mainwindows.ui 对应的 ui_mainwindow.py 文件：

from PySide2 import QtCore, QtGui, QtWidgets

class Ui_MainWindow(object):
    def setupUi(self, MainWindow):
        MainWindow.setObjectName("MainWindow")
        MainWindow.resize(800, 600)
        self.centralwidget = QtWidgets.QWidget(MainWindow)
        self.centralwidget.setObjectName("centralwidget")
        self.centralWidget = QtWidgets.QWidget(self.centralwidget)
        self.centralWidget.setGeometry(QtCore.QRect(80, 30, 421, 241))
        self.centralWidget.setObjectName("centralWidget")
        self.pushButton = QtWidgets.QPushButton(self.centralWidget)
        self.pushButton.setGeometry(QtCore.QRect(110, 80, 201, 81))
        self.pushButton.setObjectName("pushButton")
        self.menuBar = QtWidgets.QMenuBar(self.centralwidget)
        self.menuBar.setGeometry(QtCore.QRect(80, 30, 400, 20))
        self.menuBar.setObjectName("menuBar")
        self.mainToolBar = QtWidgets.QToolBar(self.centralwidget)
        self.mainToolBar.setGeometry(QtCore.QRect(80, 30, 8, 17))
        self.mainToolBar.setObjectName("mainToolBar")
        self.statusBar = QtWidgets.QStatusBar(self.centralwidget)
        self.statusBar.setGeometry(QtCore.QRect(80, 30, 3, 22))
        self.statusBar.setObjectName("statusBar")
        MainWindow.setCentralWidget(self.centralwidget)
        self.menubar = QtWidgets.QMenuBar(MainWindow)
        self.menubar.setGeometry(QtCore.QRect(0, 0, 800, 22))
        self.menubar.setObjectName("menubar")
        MainWindow.setMenuBar(self.menubar)
        self.statusbar = QtWidgets.QStatusBar(MainWindow)
        self.statusbar.setObjectName("statusbar")
        MainWindow.setStatusBar(self.statusbar)

        self.retranslateUi(MainWindow)
        QtCore.QMetaObject.connectSlotsByName(MainWindow)

    def retranslateUi(self, MainWindow):
        MainWindow.setWindowTitle(QtWidgets.QApplication.translate("MainWindow", "MainWindow", None, -1))
        self.pushButton.setText(QtWidgets.QApplication.translate("MainWindow", "PushButton", None, -1))

进而可以通过导入生成的Python类来使用UI：

import sys
from PySide2.QtWidgets import QApplication, QMainWindow
from PySide2.QtCore import QFile
from ui_mainwindow import Ui_MainWindow

class MainWindow(QMainWindow):
    def __init__(self):
        super(MainWindow, self).__init__()
        self.ui = Ui_MainWindow()
        self.ui.setupUi(self)

if __name__ == "__main__":
    app = QApplication(sys.argv)

    window = MainWindow()
    window.show()

    sys.exit(app.exec_())

或者也可以直接使用原ui文件：

import sys
from PySide2.QtUiTools import QUiLoader
from PySide2.QtWidgets import QApplication
from PySide2.QtCore import QFile

if __name__ == "__main__":
    app = QApplication(sys.argv)

    ui_file = QFile("mainwindow.ui")
    ui_file.open(QFile.ReadOnly)

    loader = QUiLoader()
    window = loader.load(ui_file)
    ui_file.close()
    window.show()

    sys.exit(app.exec_())