from tkinter import *
class Calculator:
    def __init__(self, master):
        self.operator = "+"
        self.txtOp = ""

        
        self.master = master
        master.title("Simple Calculator")

        self.f1 = Frame(master)
        self.f1.pack()
        self.lblNum1 = Label(self.f1, text = "num1:")
        self.lblNum1.pack(side = LEFT)
        self.strNum1 = StringVar()
        self.strNum1.set("")
        self.entNum1 = Entry(self.f1, textvariable = self.strNum1)
        self.entNum1.pack(side = LEFT)

        # add "+" button
        self.f2 = Frame(master)
        self.f2.pack()
        self.btnAdd = Button(self.f2, text = "+", command = self.changeOpAdd)
        self.btnAdd.pack(side = LEFT)

        self.f3 = Frame(master)
        self.f3.pack()
        self.lblNum2 = Label(self.f3, text = "num2:")
        self.lblNum2.pack(side = LEFT)
        self.strNum2 = StringVar()
        self.strNum2.set("")
        self.entNum2 = Entry(self.f3, textvariable = self.strNum2)
        self.entNum2.pack(side = LEFT)

        # add "=" button
        self.f4 = Frame(master)
        self.f4.pack()
        self.btnEqu = Button(self.f4, text = "=", command = self.calc)
        self.btnEqu.pack(side = LEFT)

        self.f5 = Frame(master)
        self.f5.pack()
        self.strResult = StringVar()
        self.strResult.set("")
        self.lblResult = Label(self.f5, textvariable = self.strResult)
        self.lblResult.pack(side = LEFT)

    def changeOpAdd(self):
        self.operator = '+'

    def calc(self):
        self.txtOp = self.strNum1.get() + self.operator + self.strNum2.get()
        self.strResult.set(str(eval(self.txtOp)))

root = Tk()
my_gui = Calculator(root)
root.mainloop()