Calcular a Taxa de Juro Simples usando tkinter

import tkinter

class compintapp_tk(tkinter.Tk):
    def __init__(self,parent):
        tkinter.Tk.__init__(self,parent)
        self.parent = parent
        self.initialize()

    def initialize(self):
        self.grid()
        ## Title  ---------------------------------------
        titlelbl = tkinter.Label(self, text="Calculadora da" 
                                "Taxa de Juro Simples",
                              anchor="center",fg="white",bg="blue")
        titlelbl.grid(column=0,row=0,columnspan=3,sticky='EW')
      ## End Title & subtitle labels ---------------------------------------

      ## Input Labels --------------------------------------------------
        principallbl = tkinter.Label(self, text="Investimento" 
                                " Inicial:",
                              anchor="w",fg="white",bg="SlateGray4")
        principallbl.grid(column=0,row=1,columnspan=1,sticky='EW')

        intratelbl = tkinter.Label(self, text="Taxa de Juro"
                                   "(% Anual):",
                              anchor="w",fg="white",bg="SlateGray4")
        intratelbl.grid(column=0,row=2,columnspan=1,sticky='EW')

        yrslbl = tkinter.Label(self, text="Números de Anos do "
                               "Investimento:",
                              anchor="w",fg="white",bg="SlateGray4")
        yrslbl.grid(column=0,row=3,columnspan=1,sticky='EW')

        amountlbl = tkinter.Label(self, text="Montante Total:",
                              anchor="w",fg="white",bg="SlateGray4")
        amountlbl.grid(column=0,row=5,columnspan=1,sticky='EW')

        earnlbl = tkinter.Label(self, text="Juros Totais:",
                              anchor="w",fg="white",bg="SlateGray4")
        earnlbl.grid(column=0,row=6,columnspan=1,sticky='EW')
      ## end of input labels ------------------------------------------------
        ## Input Boxes --------------------------------------------------------

        self.pr = tkinter.DoubleVar()
        principal = tkinter.Entry(self,textvariable=self.pr)
        principal.grid(column=1,row=1, sticky='EW')

        self.rate = tkinter.DoubleVar()
        intrate = tkinter.Entry(self,textvariable=self.rate)
        intrate.grid(column=1,row=2, sticky='EW')
        self.years = tkinter.DoubleVar()
        yrs = tkinter.Entry(self,textvariable=self.years)
        yrs.grid(column=1,row=3, sticky='EW')
      ## end of input boxes -------------------------------------------------
       ## Button
        button = tkinter.Button(self,text="Calcular",
                                command=self.OnButtonClick)
        button.grid(column=0,row=4,columnspan=3)
        ## end of button

        ## Output labels
        self.amt = tkinter.StringVar()
        amtout = tkinter.Label(self,textvariable=self.amt,
                              anchor="e",fg="red",bg="yellow")
        amtout.grid(column=1,row=5,columnspan=2,sticky='EW')

        self.earn = tkinter.StringVar()
        earnout = tkinter.Label(self,textvariable=self.earn,
                              anchor="e",fg="red",bg="gold")
        earnout.grid(column=1,row=6,columnspan=2,sticky='EW')
        ## end of output labels -----------------------------------------------

        self.grid_columnconfigure(0,weight=1)
        self.resizable(True,False)

    def OnButtonClick(self):
        p = self.pr.get()
        i = self.rate.get()
        y = self.years.get()
        t=(i/100)
        ## Simples Interest Calculation
        amount = round((p*(1+t*y)),2)
        intearn = round((amount - p),2)
        self.amt.set(amount)
        self.earn.set(intearn)

if __name__ == "__main__":
    app = compintapp_tk(None)
    app.title('Calculadora da Taxa de Juro Simples')
app.mainloop()


Adapção do código do:Osama Sidat (Código aberto)