Why does a change in nested list doesn't affect the first layers and only the subsequent layers?

Question

I just wanted to know how do we get the following output. The given below is a Python code:

x=[1,3,6,[18]]
y=list(x)
x[3][0]=15
x[1]=12
print(y)

Output is:

[1,3,6,[15]]

Why x[1]=12 didn't make any changes in the list y ? But why x[3][0]=15 changed the element in list y ? Can you give a detailed and simple explanation to this? Why ???

Answer 1

Imagine each element is a variable:

x = [a,b,c,d] # 1,3,6,[18]

you copy that as y so you have

y = [a,b,c,d] # 1,3,6,[18]

when you do x[3][0] = 15 you are changing the first element of d . notice that d is still the variable d , you changed its content but it's still the same d in both lists.

Now you change the second element of x

x = [a,e,c,d] # 1,12,6,[15]

but y is still

y = [a,b,c,d] # 1,3,6,[15]