main.py

# from PyQt5.QtWidgets import QApplication, QLabel

# app = QApplication([])
# label = QLabel('Hello World!')
# label.show()
# app.exec_()


# from PyQt5.QtWidgets import QApplication, QWidget, QPushButton, QVBoxLayout
# app = QApplication([])
# window = QWidget()
# layout = QVBoxLayout()
# app.setStyleSheet("QPushButton { margin: 10ex; }")

# layout.addWidget(QPushButton('Top'))
# layout.addWidget(QPushButton('Bottom'))
# window.setLayout(layout)
# window.show()
# app.exec_()


from PyQt5 import QtCore, QtGui, QtWidgets
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))

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

        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("")

        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
        self.label.setText("You clicked PushButton")

        # Hiding pushbutton from the main window
        # after button get clicked.
        self.pushButton.hide()


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

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

    sys.exit(app.exec_())