Creating the GUI as a class (3)

        # create frame to hold the middle row of widgets:
        rframe = Frame(top)
        # this frame (row) is packed from top to bottom:
        rframe.pack(side='top', padx=10, pady=20)

        # create label and entry in the frame and pack from left:
        r_label = Label(rframe, text='The sine of')
        r_label.pack(side='left')

        self.r = StringVar() # variable to be attached to r_entry
        self.r.set('1.2')    # default value
        r_entry = Entry(rframe, width=6, textvariable=self.r)
        r_entry.pack(side='left')
        r_entry.bind('<Return>', self.comp_s)

        compute = Button(rframe, text=' equals ',
                         command=self.comp_s, relief='flat')
        compute.pack(side='left')