Calcular o Custo do Capital usando tkinter

import tkinter

class capmapp_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="Rentabilidade "
                                            "Esperada "
                                            "para o Património "
                                            "Líquido",
                              anchor="center",fg="white"
                                 ,bg="blue")
        titlelbl.grid(column=0,row=0,columnspan=3,
                      sticky='EW')

      ## Input Labels --------------------------------------------------
        rflbl = tkinter.Label(self, text="Rentabilidade "
                                               "do ativo livre"
                                         " de risco "
                                         "(em%): ",
                              anchor="w",fg="white",
                                    bg="SlateGray4")
        rflbl.grid(column=0,row=1,columnspan=1,
                         sticky='EW')

        betalbl = tkinter.Label(self, text="Índice beta"
                                           " do património "
                                           "líquido:",
                              anchor="w", fg="white",
                              bg="SlateGray4")
        betalbl.grid(column=0, row=2, columnspan=1,
                   sticky='EW')

        rmlbl = tkinter.Label(self, text="Rentabilidade esperado"
                                         " sobre"
                                         " o índice de mercado "
                                         "(em %):",
                                    anchor="w", fg="white",
                                     bg="SlateGray4")
        rmlbl.grid(column=0, row=3, columnspan=1,
                          sticky='EW')

        ralbl = tkinter.Label(self, text="Rentabilidade esperado "
                                              " para o"
                                         " património líquido"
                                         "(em %)",
                                    anchor="w", fg="white",
                                    bg="green")
        ralbl.grid(column=0, row=5, columnspan=1,
                         sticky='EW')


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

        self.rf = tkinter.DoubleVar()
        riskfree = tkinter.Entry(self,textvariable=self.rf)
        riskfree.grid(column=1,row=1, sticky='EW')
        self.bt = tkinter.DoubleVar()
        beta = tkinter.Entry(self, textvariable=self.bt)
        beta.grid(column=1, row=2, sticky='EW')

        self.rm = tkinter.DoubleVar()
        riskmarket = tkinter.Entry(self, textvariable=self.rm)
        riskmarket.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.ra = tkinter.StringVar()
        rendativo = tkinter.Label(self,
                                  textvariable=self.ra,
                              anchor="e",fg="red",
                                  bg="yellow")
        rendativo.grid(column=1,row=5,columnspan=2,
                       sticky='EW')

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

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

    def OnButtonClick(self):
        a = self.rf.get()
        b = self.bt.get()
        c = self.rm.get()
        ap=a/100
        cp=c/100
        rentibilidaativo=ap+b*(cp-ap)
        rapercentagem=rentibilidaativo*100
        rentibilidaativof = round(rapercentagem,2)
        self.ra.set(rentibilidaativof)

if __name__ == "__main__":
    app = capmapp_tk(None)
    app.title('CAPM')
app.mainloop()