使用Qt Designer设计水平布局，加addStretch()不起作用，解决办法？

解决办法：
将main函数中的MainWindow.show()改为：ui.show()

if __name__=='__main__':
  app = QApplication(sys.argv)
  MainWindow = QMainWindow()
  ui = Ui_MainWindow()
  ui.setupUi(MainWindow)
  ui.show()
  app.exec()

完整代码：

import  sys

from PyQt5 import QtCore, QtGui, QtWidgets
from PyQt5.QtWidgets import QApplication, QWidget, QHBoxLayout, QPushButton,QMainWindow

class Ui_MainWindow(QWidget):
    def __init__(self):
        super().__init__()
        self.setupUi(MainWindow)

    def setupUi(self,MainWindow):
        MainWindow.setObjectName("MainWindow")
        MainWindow.resize(800, 600)
        self.centralwidget = QtWidgets.QWidget(MainWindow)
        self.centralwidget.setObjectName("centralwidget")
        self.horizontalLayoutWidget = QtWidgets.QWidget(self.centralwidget)
        self.horizontalLayoutWidget.setGeometry(QtCore.QRect(290, 160, 320, 80))
        self.horizontalLayoutWidget.setObjectName("horizontalLayoutWidget")
        self.horizontalLayout = QtWidgets.QHBoxLayout(self.horizontalLayoutWidget)
        self.horizontalLayout.setContentsMargins(0, 0, 0, 0)
        self.horizontalLayout.setObjectName("horizontalLayout")
        self.pushButton = QtWidgets.QPushButton(self.horizontalLayoutWidget)
        self.horizontalLayout.addStretch(1)
        self.pushButton.setObjectName("pushButton")
        self.horizontalLayout.addWidget(self.pushButton)
        self.pushButton_2 = QtWidgets.QPushButton(self.horizontalLayoutWidget)
        self.pushButton_2.setObjectName("pushButton_2")
        self.horizontalLayout.addStretch(2)
        self.horizontalLayout.addWidget(self.pushButton_2)
        self.pushButton_3 = QtWidgets.QPushButton(self.horizontalLayoutWidget)
        self.pushButton_3.setObjectName("pushButton_3")
        self.horizontalLayout.addStretch(1)
        self.horizontalLayout.addWidget(self.pushButton_3)
        self.pushButton_4 = QtWidgets.QPushButton(self.horizontalLayoutWidget)
        self.pushButton_4.setObjectName("pushButton_4")
        self.horizontalLayout.addStretch(1)
        self.horizontalLayout.addWidget(self.pushButton_4)
        MainWindow.setCentralWidget(self.centralwidget)
        self.menubar = QtWidgets.QMenuBar(MainWindow)
        self.menubar.setGeometry(QtCore.QRect(0, 0, 800, 23))
        self.menubar.setObjectName("menubar")
        MainWindow.setMenuBar(self.menubar)
        self.statusbar = QtWidgets.QStatusBar(MainWindow)
        self.statusbar.setObjectName("statusbar")
        MainWindow.setStatusBar(self.statusbar)
        self.setLayout(self.horizontalLayout)
        self.retranslateUi(MainWindow)
        QtCore.QMetaObject.connectSlotsByName(MainWindow)

    def retranslateUi(self, MainWindow):
        _translate = QtCore.QCoreApplication.translate
        MainWindow.setWindowTitle(_translate("MainWindow", "MainWindow"))
        self.pushButton.setText(_translate("MainWindow", "按钮1"))
        self.pushButton_2.setText(_translate("MainWindow", "按钮2"))
        self.pushButton_3.setText(_translate("MainWindow", "按钮3"))
        self.pushButton_4.setText(_translate("MainWindow", "按钮4"))



if __name__=='__main__':
  app = QApplication(sys.argv)
  MainWindow = QMainWindow()
  ui = Ui_MainWindow()
  ui.setupUi(MainWindow)
  ui.show()
  app.exec()