Making Gridded Widgets Expandable

[Jim]

Hi,
I'm looking at page 548 of Programming Python (3rd Edition) by Mark
Lutz.
The following GUI script works with no problem, i.e., the rows and
columns expand:
=================================================================
# Gridded Widgets Expandable page 548

from Tkinter import *
colors = ["red", "white", "blue"]

def gridbox(root):
    Label(root, text = 'Grid').grid(columnspan = 2)
    r = 1
    for c in colors:
        l = Label(root, text=c, relief=RIDGE, width=25)
        e = Entry(root, bg=c,   relief=SUNKEN, width=50)
        l.grid(row=r, column=0, sticky=NSEW)
        e.grid(row=r, column=1, sticky=NSEW)
        root.rowconfigure(r, weight=1)
        r += 1
    root.columnconfigure(0, weight=1)
    root.columnconfigure(1, weight=1)

root = Tk()
gridbox(Toplevel(root))
Button(root, text="Quit", command=root.quit).grid()
mainloop()
=================================================================
However, the following GUI script using class does not expand rows and
columns:
=================================================================
# Gridded Widgets Expandable 2

from Tkinter import *
colors = ["red", "white", "blue"]

class GUI(Frame):
    def __init__(self,master):
        Frame.__init__(self,master)
        self.grid()
        self.gridbox()

    def gridbox(self):
        Label(self, text = 'Grid').grid(columnspan = 2)
        r = 1
        for c in colors:
            l = Label(self, text=c, relief=RIDGE, width=25)
            e = Entry(self, bg=c,   relief=SUNKEN, width=50)
            l.grid(row=r, column=0, sticky=NSEW)
            e.grid(row=r, column=1, sticky=NSEW)
            self.rowconfigure(r, weight=1)
            r += 1
        self.columnconfigure(0, weight=1)
        self.columnconfigure(1, weight=1)

root = Tk()
root.title("Gridded Widgets Expandable")
app = GUI(root)
Button(root, text="Quit", command=root.quit).grid()
root.mainloop()
=================================================================
What am I missing?
Thanks,
Jim