Calcular o Preço com o Desconto usando tkinter

import tkinter

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

    def initialize(self):
        self.grid()
        ## Titulo  ---------------------------------------
        titlelbl = tkinter.Label(self, text="Calcular "
                                            "Preço "
                                            "com Desconto ",
                              anchor="center",fg="black"
                                 ,bg="red",font=('Arial', 13,'bold'))
        titlelbl.grid(column=0,row=0,columnspan=3,
                      sticky='EW')

      ## Inputs --------------------------------------------------
        precoiniciallbl = tkinter.Label(self, text="Preço Inicial (€): ",
                              anchor="center",fg="dodger blue",
                                    bg="snow",font=('Helvetica', 10,'italic'))
        precoiniciallbl.grid(column=0,row=1,columnspan=1,
                         sticky='EW')

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


#--------------------------------------------------------------------------------
        perdesclbl = tkinter.Label(self, text="Percentagem do "
                                           " Desconto (em %)",
                                   anchor="center", fg="dodger blue",
                                   bg="snow", font=('Helvetica', 10, 'italic'))
        perdesclbl.grid(column=0, row=2, columnspan=1,
                   sticky='EW')

        self.dec = tkinter.DoubleVar()
        txdesconto = tkinter.Entry(self, textvariable=self.dec)
        txdesconto.grid(column=1, row=2, sticky='EW')
#------------------------------------------------------------------------------
        pflbl = tkinter.Label(self, text="Preço com o "
                                              " Desconto    ====>",
                              anchor="center", fg="dodger blue",
                              bg="snow", font=('Courier New', 10, 'bold '))
        pflbl.grid(column=0, row=4, columnspan=1,
                         sticky='EW')



       ## Button
        button = tkinter.Button(self, text="Calcular",width=25,bg="dodger blue",
                                command=self.OnButtonClick)
        button.grid(column=0,row=3,columnspan=3)
        ## end of button

        ## Output labels
        self.pf = tkinter.StringVar()
        precofinal = tkinter.Label(self,
                                  textvariable=self.pf,
                              anchor="center",fg="white",
                                  bg="navy")
        precofinal.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):
        a = self.po.get()
        b = self.dec.get()
        decp=b/100
        novopreco = a-(a*decp)
        pref = round(novopreco,2)
        self.pf.set(pref)

if __name__ == "__main__":
    app = cpdpp_tk(None)
    app.title('Calcular o Preço com Desconto')
app.mainloop()