Python: When do two variables point at the same object in memory?

Question

Here is an example:

l = [1, 5, 9, 3]
h = l

h[0], h[2] = h[2], h[0]

print(h) # [9, 5, 1, 3]
print(l) # [9, 5, 1, 3]

h = h*2
print(h) # [9, 5, 1, 3, 9, 5, 1, 3]
print(l) # [9, 5, 1, 3]

My understanding was that calling setting h = l would simply point h at the same item in memory that l was pointing at. So why is it that in the last 3 lines, h and l don't give the same results?

Answer 1

That's quite simple to check, run this simple test:

l = [1, 5, 9, 3]
h = l

h[0], h[2] = h[2], h[0]

print(h)  # [9, 5, 1, 3]
print(l)  # [9, 5, 1, 3]

print id(h), id(l)
h = h * 2
print id(h), id(l)

print(h)  # [9, 5, 1, 3, 9, 5, 1, 3]
print(l)  # [9, 5, 1, 3]

As you can see because of the line h = h * 2, the h's id has been changed

Why is this? When you're using * operator it creates a new list (new memory space). In your case this new list is being assigned to the old h reference, that's why you can see the id is different after h = h * 2

If you want to know more about this subject, make sure you look at Data Model link.

Answer 2

The assignment does make h point to the same item as l. However, it does not permanently weld the two. When you change h with h = h * 2, you tell Python to build a doubled version elsewhere in memory, and then make h point to the doubled version. You haven't given any instructions to change l; that still points to the original item.