A list changes when another list is changed

[JeppeDruedahl]

Why is the first element of list A equal to zero when I run this code?

A = [1,2,3] 
B = A 
B[0] = 0
print(A[0])

[JeppeDruedahl]

The variable A is a reference to the list [1,2,3]. The second line, B = A, copies this reference, not the list itself. A and B therefore now refer to the same list.