Выдаёт ошибку AttributeError: 'NoneType' object has no attribute 'update'

хотел сделать простенькую систему двумерного перемещения, но в генераторе пространства выдаёт AttributeError: 'NoneType' object has no attribute 'update' на 36 строке

`

 #player
p_pos_x = 0
p_pos_y = 0
#modules
from random import *
#max
max_pos_x = 3
max_pos_y = 3
max_pos_x_ = -3
max_pos_y_ = -3
#playground
pg = {}
#gen
pos = -3
pos_y = -3
num = 0
y_o = {}
y_t = {}
y_th = {}
y_f = {}
y_fi = {}
y_s = {}
y_se = {}
while True:
    num += 1
    dot = {num: pos}
    pos += 1
    if pos == max_pos_x_:
        pos = -3
        num = 0
        pos_y += 1
        if pos_y == max_pos_y:
            break
    if pos_y == -3:
        y_o = y_o.update(dot)
    elif pos_y == -2:
        y_t = y_t.update(dot)
    elif pos_y == -1:
        y_th = y_th.update(dot)
    elif pos_y == 0:
        y_f = y_f.update(dot)
    elif pos_y == 1:
        y_fi = y_fi.update(dot)
    elif pos_y == 2:
        y_s = y_s.update(dot)
    elif pos_y == 3:
        y_se = y_se.update(dot)
#screen
print(y1)

`