how come the output is 12, I still don't really understand how y[0] value was also changed

Question

y=[5]
z=y
z[0]+=1
print(y[0]+z[0])
#prints: 12

Answer 1

When you say z=y you are not creating a new copy of the list [5]. You are only creating a second name (z) that refers to the same list object as the first name (y). So y[0] and z[0] refer to the same thing, not only the same number originally but the same memory location. If you change z[0] you also change y[0] because they are two references to the same object.

Answer 2

Jay Matthews but how y[0] increments too?