Calcular o preço final de um produto

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="Calcular o "
                                            "Preço Final",
                              anchor="center",fg="white"
                                 ,bg="blue")
        titlelbl.grid(column=0,row=0,columnspan=3,
                      sticky='EW')
      ## End Title & subtitle labels ---------------------------------------

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

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

        pfinallbl = tkinter.Label(self, text="Preço Final:",
                              anchor="w",fg="white",
                                  bg="SlateGray4")
        pfinallbl.grid(column=0,row=4,columnspan=1,
                       sticky='EW')

      ## end of input labels ------------------------------------------------
        ## Input Boxes --------------------------------------------------------

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

        self.txm = tkinter.DoubleVar()
        taxa = tkinter.Entry(self,textvariable=self.txm)
        taxa.grid(column=1,row=2, sticky='EW')

      ## end of input boxes -------------------------------------------------
       ## Button
        button = tkinter.Button(self,text="Calcular",
                                command=self.OnButtonClick)
        button.grid(column=0,row=3,columnspan=3)
        ## end of button

        ## Output labels
        self.pfinal = tkinter.StringVar()
        prodfinal = tkinter.Label(self,
                                  textvariable=self.pfinal,
                              anchor="e",fg="red",
                                  bg="yellow")
        prodfinal.grid(column=1,row=4,columnspan=2,
                       sticky='EW')

        ## end of output labels -----------------------------------------------

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

    def OnButtonClick(self):
        p = self.cp.get()
        i = self.txm.get()
        t=(i/100)
        ## Compound Interest Calculation
        prodfinal = round((p/(1-t)),2)
        self.pfinal.set(prodfinal)

if __name__ == "__main__":
    app = compintapp_tk(None)
    app.title('Calcular o Preço Final')
app.mainloop()