I do not understand why this code does not work. Could someone explain it to me?

Question

#brainf**k interpreter

import sys

f = open(sys.argv[1], "r")
point = f.read()
f.close()

pointPos = 0

cell = [0]

cellPos = 0

while pointPos < len(point):

if point[pointPos] == ">":

cellPos += 1

if len(cell)> cellPos:

cell.append(0)

elif point[pointPos] == "<":

cellPos -= 1

if cellPos < 0:

print("Error: Moved off tape!")

sys.exit(0)

elif point[pointPos] == "+":

cell[cellPos] += 1

if cell[cellPos] >= 255:

cell[cellPos] = 0

elif point[pointPos] == "-":

cell[cellPos] -= 1

if cell[cellPos] <= -1:

cell[cellPos] = 255

elif point[pointPos] == ".":

print(chr(cell[cellPos]), end = "")

elif point[pointPos] == ",":

inp = input()

cell[cellPos] = ord(imp[0])

elif point[pointPos] == "[":

if cell[cellPos] == 0:

countOpened = 0

point[pointPos] += 1

while pointPos < len(prg):

if point[pointPos] == "]" and countOpened == 0:
break

elif point[pointPos] == "[":

countOpened += 1

elif point[pointPos] == "]":

countOpened -= 1

pointPos += 1

elif point[pointPos] == "]":

if cell[cellPos] != 0:

countOpened = 0

pointPos -= 1

while pointPos > 0:

if point[pointPos] == "[" and countOpened == 0:

break

elif point[pointPos] == "]":

countOpened += 1

elif point[pointPos] == "[":

countOpened -= 1

pointPos -= 1