[Python] the code does not give the results what I want using equal sign in python

I have code like this:

import numpy as np
a = np.zeros(shape = (4,4))
a+= 2
b = np.zeros(shape = (4,4))
b+=2
t = 0
while t<2:
for i in range(1,3):
for j in range(1,3):
if a[i,j] == a[i-1,j]:
b[i,j] = a[i,j]+1
print(a,t)
print(b,t)
a = b
t+= 1


I am hoping that at t = 2 a = [2 2 2 2, 2 3 3 2, 2 3 3 2, 2 2 2 2] and b = [2 2 2 2, 2 3 3 2, 2 4 4 2, 2 2 2 2] but in fact at the end of the run the a = [2 2 2 2, 2 3 3 2, 2 4 4 2, 2 2 2 2]

Anyone know why? Is it because I am declaring a = b? If it is, is there any way to do it?

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