Why am I getting KeyError: 0

Question

I am attempting to code Conway's Game of Life. As far as I can tell the program should be complete, but when I run it I get KeyError: 0, which to my understanding means a nonexistent dictionary key was referenced. The dictionary (cell_dict) contains the 20x20 grid of tkinter buttons that form the dead/alive cells.

Here is the whole code:

#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()

I notice that once the key error pops, i = 399 and j is undefined. So it seems like the code is somehow skipping the "for i in range(0,2): for j in range(0,20):". I'm new to coding so I'm not sure where I went wrong.

Answer 1

You didn't include the traceback, so this is just an educated guess, but I suspect your issue is more cleanly written like this:

cell_dict = {}
for i in range(0,400):
    cell_dict[(int(i/20)%20,i%20)] = None

print(cell_dict)
for i in range(0,20):
    for j in range(0,20):
    cell = cell_dict[i][j]  # key error happens here

And in fact you do have your key error there.

Your keys are cell_dict[(x, y)] -> cell, but you meant to have cell_dict[x][y] -> cell:

You can fix that in several ways, allocating dicts in your loop, using setdefault as below, or collections.defaultdict, etc.

    cell_dict.setdefault(int(i/20)%20, {})[i%20] = None

Answer 2

it looks like you store cells locations as a tuple of two integers. so you should address cell_dict with that instead of an integer.

Use cell_dict[(i, j)] instead of cell_dict[i][j].