SetText в PyQt

from PyQt5.QtWidgets import QTextEdit, QApplication,QListWidget, QWidget, QPushButton, QToolTip, QMessageBox, QLineEdit, QLabel, QGridLayout, QInputDialog
from PyQt5.QtGui import QFont
from PyQt5.QtCore import QCoreApplication
from PyQt5 import QtCore, QtGui, QtWidgets
class Window(QtWidgets.QMainWindow):
    def __init__(self):
        QtWidgets.QMainWindow.__init__(self)
        self.setupUi(self)
    def setupUi(self, MainWindow):
        MainWindow.setObjectName("MainWindow")
        MainWindow.resize(574, 311)
        self.centralwidget = QtWidgets.QWidget(MainWindow)
        self.centralwidget.setObjectName("centralwidget")
        self.returnt = QtWidgets.QLineEdit(self.centralwidget)
        self.returnt.setGeometry(QtCore.QRect(180, 110, 171, 41))
        self.returnt.setObjectName("returnt")
        self.beg = QtWidgets.QPushButton(self.centralwidget)
        self.beg.setGeometry(QtCore.QRect(410, 210, 75, 23))
        self.beg.setObjectName("beg")
        self.beg.clicked.connect(self.buttonClicked)
        MainWindow.setCentralWidget(self.centralwidget)
        self.menubar = QtWidgets.QMenuBar(MainWindow)
        self.menubar.setGeometry(QtCore.QRect(0, 0, 574, 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)
        QtCore.QMetaObject.connectSlotsByName(MainWindow)
    def retranslateUi(self, MainWindow):
        _translate = QtCore.QCoreApplication.translate
        MainWindow.setWindowTitle(_translate("MainWindow", "MainWindow"))
        self.beg.setText(_translate("MainWindow", "Начать"))
    def buttonClicked(self):
        for i in range(5):
            self.returnt.setText(str(i))
            
app = QtWidgets.QApplication(sys.argv)
ex = Window()
ex.show()
sys.exit(app.exec_())