main_gui.py

from PyQt5 import QtCore, QtGui, QtWidgets
from Functions import Add
import sys


class Ui_MainWindow(object):

    def setupUi(self, MainWindow):
        MainWindow.resize(506, 312)
        self.centralwidget = QtWidgets.QWidget(MainWindow)

        # adding pushbutton
        self.pushButton = QtWidgets.QPushButton(self.centralwidget)
        self.pushButton.setGeometry(QtCore.QRect(200, 150, 93, 28))

        self.label = QtWidgets.QLabel(self.centralwidget)
        self.label.setGeometry(QtCore.QRect(140, 90, 221, 20))

        # keeping the text of label empty before button get clicked
        self.label.setText("")

        # adding signal and slot
        self.pushButton.clicked.connect(self.changelabeltext)

        MainWindow.setCentralWidget(self.centralwidget)
        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", "Push Button"))

    def changelabeltext(self):
        # changing the text of label after button get clicked
        Add.changelabeltext(self)


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

    MainWindow = QtWidgets.QMainWindow()
    ui = Ui_MainWindow()
    ui.setupUi(MainWindow)
    MainWindow.show()

    sys.exit(app.exec_())