为什么我收到KeyError：0

我正在尝试编写Conway的《人生游戏》。据我所知，程序应该是完整的，但是当我运行它时，我得到KeyError：0，据我所知，这意味着引用了不存在的字典键。字典（cell_dict）包含20x20的tkinter按钮网格，这些按钮构成了无效/有效单元格。

这是完整的代码：

#imports
import tkinter
import random

#functions
def step(): #updates the status of each cell
    for i in range(0,20): # runs through each cell to check the number of living neighbors
        for j in range(0,20):

            cell = cell_dict[i][j] #key error happens here
            pop = 0

            for n in range(-1,2): #checks each neighbor and tracks the number of alive ones
                for m in range(-1,2):
                    if i+n in range(0,20) and j+m in range(0,20):
                        if n != 0 or m != 0:
                            cell_neighbor = cell_dict[i+n][j+m]
                            if cell_neighbor['bg'] == "yellow":
                                pop += 1

            if cell['bg'] == "yellow" and (pop == 2 or pop == 3): #primes the new status for the cell
                cell['fg'] = "yellow"
            elif cell['bg'] == "black" and pop == 3:
                cell['fg'] = "yellow"
            else:
                cell['fg'] = "black"

    for i in range(0,20): #updates the status of each cell
        for j in range(0,20):
            cell = cell_dict[i][j]
            if cell['fg'] == "black":
                cell['bg'] = "black"
            elif cell['fg'] == "yellow":
                cell['bg'] = "yellow"

    window.after(1000,step) #repeats the process every second

#gui
window = tkinter.Tk()

cell_dict = {} #primes the dictionary

for i in range(0,400): #creates all the cells
    cell = tkinter.Button(fg = "white", bg = "white", height = 1, width = 2)

    r = random.randrange(0,2) #randomly determines if the cell will be initially dead or alive

    if r == 0:
        cell['bg'] = "black" #dead
    elif r == 1:
        cell['bg'] = "yellow" #alive

    cell.grid(row = int(i/20)%20, column = i%20) #packs the cell into the grid

    cell_dict[(int(i/20)%20,i%20)] = cell #stores the cell in the dictionary

#mainloop
step()
window.mainloop()

我注意到，一旦出现关键错误，i = 399并且j未定义。因此，看起来代码似乎以某种方式跳过了“对于范围（0,2）中的i：对于范围（0,20）中的j：”。我是编码的新手，所以我不确定哪里出错了。