why did one value change but the second value did not?

Question

a = [ 1, 2 ]
b = a
a.append(3)
print(b) # shows [ 1 2 3 ] which means b changed

c = 4
d = c
c = 8
print(d) # shows 4 which means d did not change

Why

Answer 1

The two examples are not equivalent.

By doing b = a you are telling b to point to the same list that a points to. If you change the list through a it will be changed even if introspected through b. There is only ever one list in memory.

In the second example you are doing d = c which tells d to point to the same integer that c does, but then you are telling c to point to another integer. d does not know about it, and it is still pointing to the same integer that c used to point to.

The equivalent example using lists to your second example will be

a = [1, 2]
b = a
a = []
print(a)
# []
print(b)
# [1, 2]

Check these visualizations:

Your first example

Your second example

My example