Pyqt5，类函数不会更改标签文本

import sys
from PyQt5.QtWidgets import QApplication, QLabel, QPushButton, QVBoxLayout, QWidget

class MainWindow(QWidget):
    def __init__(self):
        super().__init__()
        self.initUI()

    def initUI(self):
        self.label = QLabel("初始文本", self)

        button = QPushButton("更改文本", self)
        button.clicked.connect(self.changeText)

        layout = QVBoxLayout()
        layout.addWidget(self.label)
        layout.addWidget(button)

        self.setLayout(layout)
        self.setWindowTitle("PyQt5示例")
        self.show()

    def changeText(self):
        self.label.setText("新的文本")

if __name__ == '__main__':
    app = QApplication(sys.argv)
    mainWindow = MainWindow()
    sys.exit(app.exec_())