import sys
from PyQt5 import QtWidgets

def window():
    app = QtWidgets.QApplication(sys.argv)
    w = QtWidgets.QWidget()
    b = QtWidgets.QLabel(w)
    b.setText("Hello World!")
    w.setGeometry(500, 500, 300, 100)
    b.move(110, 40)
    w.setWindowTitle("My first PyQt App ;)")
    w.show()
    sys.exit(app.exec_())

if __name__ == '__main__':
    window()

import sys
from PyQt5 import QtWidgets, uic, QtGui


class MyWindow(QtWidgets.QDialog):
    def __init__(self):
        super(MyWindow, self).__init__()
        uic.loadUi('calculadora.ui', self)
        self.oper.addItems(["Suma","Resta","Producto",
        		    "Division"])
        self.opButton.clicked.connect(self.operar)
        self.show()

    def operar(self):
        n1 = int(self.num1.text())
        n2 = int(self.num2.text())
        op = self.oper.currentText()
        re = self.operacion(n1, n2, op)
        self.res.setText("Resultado: " + str(re))

    def operacion(self, n1, n2, op):
        if op == "Suma":
            re = n1 + n2
        elif op == "Resta":
            re = n1 - n2
        elif op == "Producto":
            re = n1 * n2
        elif op == "Division":
            if n2 == 0:
                re = " OPERACIÓN NO SE PUEDE REALIZAR"
            else:
                re = n1 / n2
        return re

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