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


class MainWindow(QWidget):
    def __init__(self):
        super().__init__()
        self.setWindowTitle('Text Changed Events')
        
        layout = QVBoxLayout()
        self.setLayout(layout)
        
        self.label = QLabel()
        layout.addWidget(self.label)
        
        self.textbox = QLineEdit()
        self.textbox.textChanged.connect(self.on_text_changed)
        layout.addWidget(self.textbox)
        
    def on_text_changed(self, text):
        self.label.setText(f'Text changed to: {text}')

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

Результат: