Python slice shows same id location

Question

I'm total beginner to Python, so please could you explain me why the following situation happens. Consider the following code:

>>> A = [1, 2, 3, 4]
>>> B = A[0:2]
>>> print id(A) == id(B)
False
>>> print id(A[0]) == id(B[0])
True                              #Why?
>>> A[0] = 9
>>> A
[9, 2, 3, 4]
>>> B 
[1, 2]
>>> print id(A[0]) == id(B[0])
False                             #Contradiction?

As you can see from the code above, I slice the list A and copy it to B, but, why print id(A[0]) == id(B[0]) evalutes Trueon the first one but the opposite when I change either of A or B's value?

Answer 1

When you do B = A[0:2], that ends up essentially doing this, as part of it: B[0] = A[0]. So the object (the integer 1) in A[0] is the same object which is in B[0].

When you set A[0] = 9, then those objects are no long the same.

Also, as @ŁukaszRogalski pointed out CPython caches small integers. So we've got A[0] == 1 == B[0], and id(1) == id(1).

When A[0] == 9, then 9 != 1 == B[0], and id(9) != id(1).