References, Objects, and Aliasing

Two References Pointing to Two Separate Objects

In this example, we have created two lists, list1 and list2. Since the two lists are created separately, list1 and list2 contain separate references to two different lists. Although the values in the list are the same, their memory addresses are different.

list1 = [1, 2, 3]

list2 = [1, 2, 3]

list1[0] = 5

print("list1 =", list1)
print("list2 =", list2)

The above code prints the following. Notice that changing the value at index 0 for list1 only changed list1’s list object. This shows that list1 and list2 contain references to separate objects.

list1 = [5, 2, 3]
list2 = [1, 2, 3]

Two References Pointing to the Same Object

The following is an example of two references pointing to the same object—these are aliased references. (An ‘alias’ is another name for something, from the Latin word alius, meaning “other”.) The first line creates the list object and list1 now stores the reference to the newly created list object. The second line copies list1’s reference and stores it into list2. In this case, list1 and list2’s references are pointing to the same object.

list1 = [1, 2, 3]
list2 = list1

print("Before modification...")
print("list1 =", list1)
print("list2 =", list2)

list2[0] = 5

print("After modification...")
print("list1 =", list1)
print("list2 =", list2)

The above code prints the following. When we update the value at index 0 for list2, notice that list1 has also been updated. This happened because list1 and list2 contain references to the same list object. Try this out in IDLE and modify the elements in list1 and list2. What happened? Are the two lists always the same?

Before modification...
list1 = [1, 2, 3]
list2 = [1, 2, 3]
After modification...
list1 = [5, 2, 3]
list2 = [5, 2, 3]