Tkinter initializing method in class

I created a simple program that displays 28 numbers on the screen. Currently, nothing happens when you click on the numbers. Eventually, I want the numbers to disappear on click. Also, when you click on a '5', you get 5 points. I would like to make this multiplayer too where the user with the most points after 10 seconds wins. But that is all for later.

from Tkinter import *
import random

class Game(Frame):

    def __init__(self, parent):
        Frame.__init__(self, parent, background="white")
        self.parent = parent
        self.parent.title("Centered window")
        self.pack(fill=BOTH, expand=1)
        self.centerWindow()

    def centerWindow(self):

        w = 850
        h = 530

        sw = self.parent.winfo_screenwidth()
        sh = self.parent.winfo_screenheight()

        x = (sw - w)/2
        y = (sh - h)/2
        self.parent.geometry('%dx%d+%d+%d' % (w, h, x, y))

def main():

    global tile_frame_column  # the labels that go on the screen of individual numbers
    tile_frame_column = {}

    global tiles_make_number  # dictionary of all the individual numbers
    tiles_make_number = {}

    global tile_frame_column_counter
    tile_frame_column_counter=0

    global tiles_make_number_counter
    tiles_make_number_counter=0

    global xpos
    xpos=35

    global ypos
    ypos=50

    tile_numbers = ['1', '2', '3', '4', '5', '6', '1', '2', '1', '2', '1', 'l', '1', '3', '4', '2', '1', '2', '1', '1', '2', '1', '1', '6', '4', '3', '2', '2']

    root = Tk()
    ex = Game(root)

    def add_number_to_screen():
        global tile_frame_column_counter
        global tiles_make_number_counter
        global tile_frame_column
        global xpos
        global ypos

        if not tile_numbers:
            return

        rand = random.choice(tile_numbers)
        tile_frame_column[tile_frame_column_counter] = Button(root, text=rand, font="Helvetica 16 bold")

        tile_frame_column[tile_frame_column_counter].place(x=xpos, y=ypos)
        tile_numbers.remove(rand)  # remove that tile from list of tiles
        xpos += 80
        if (len(tiles_make_number) % 7 == 0) & (len(tiles_make_number) > 0):
            xpos = 35
            ypos += 80
            tile_frame_column[tile_frame_column_counter].place(x=xpos, y=ypos)
            xpos += 80
        tiles_make_number[tiles_make_number_counter] = rand
        tile_frame_column_counter += 1
        tiles_make_number_counter += 1
        root.after(10, add_number_to_screen)

    root.after(10, add_number_to_screen)
    root.mainloop()


if __name__ == '__main__':
    main()

I want the main() function to be initialized, though. Whenever a game object is created I want it to automatically add the main function -- meaning add numbers to the screen. I think the way my code is now, when a game object is created it only creates the screen. There must be a better way to maintain this code, though.