[Tutor] Tkinter frame background colour

[Phil]

My original GUI worked but it was a bit of a mess so I've spent most of 
the day building a new interface to my sudoku solver. However, there is 
one problem. I used to have a red line after the third and sixth column 
of boxes, the same with the rows. I achieved this by setting the frame's 
background to red and placing some padding after the third and sixth 
group of cells so that the red background would show through and show a 
red line between the 3 x 3 boxes. Otherwise, I'm happy with the result.

import tkinter as tk

class Root(tk.Tk):
     def __init__(self):
         super().__init__()

         self.title('Sudoku Solver')
         self.geometry('300x300')

         self.frame = tk.Frame()
         self.frame.pack(expand=True)
         self.frame.bg="red"

         self.entry = [None] * 81
         for i in range(81):
             r, c = divmod(i, 9)
             self.entry[i] = tk.Entry(self.frame, width=2, 
justify=tk.CENTER, fg="red")
             #if i == i % 3:
             pad_x = (4, 0)
             self.entry[i].grid(row=r, column=c, padx=pad_x)

         self.control_frame = tk.Frame()
         self.control_frame.pack(side=tk.TOP, pady=5)
         self.b_quit = tk.Button(self.control_frame, text='Quit',
             command=self.quit)
         self.b_quit.pack(side=tk.LEFT)

         self.b_solve = tk.Button(self.control_frame, text='Solve',
             command=self.solve)
         self.b_solve.pack(side=tk.RIGHT)

     def quit(self):
         self.destroy()

     def solve(self):
         self.entry[2].insert(0, '9')
         print(self.entry[2].get())


if __name__ == '__main__':
     root = Root()
     root.mainloop()

-- 
Regards,
Phil