Mutable and immutable objects

The result of the code below is probably what you expect.

a = 3
b = a
print(a, b)
b = b + 9
print(a, b)

3 3
3 12

Here is similar code that uses lists instead, but the result might surprise you.

a = [3, 4, 5]
b = a
print(a, b)
b.append(6)
print(a, b)

[3, 4, 5] [3, 4, 5]
[3, 4, 5, 6] [3, 4, 5, 6]

Notice that we changed b by appending 6, yet the value of a has also changed. In order to understand these examples, we need to cover the concept of "variables" in Python and the concept of mutability.

The example above shows that a has to be linked in some way to b beyond just initially sharing values. In other words, assigning a to b doesn't just simply copy over the values of a. So what is happening?

Variables can be thought of as names that are attached to a value. A name itself is not the value. For example, a above is not a list. At that point in time, it refers to a particular list.

After we assign b = a, now b is another name that attached to the same list.

The above diagrams should make it clear what is happening in the example with lists. The value of a changes when b is modified because they are referring to the same thing.

The logic above explains why b changes along with a, but why didn't the example with integers behave the same way? This has to do with an important difference between integers and lists: lists are mutable while integers are not. Being mutable means that the value can be changed in-place. Let's walk through the integer scenario in diagrams.

After both a is assigned to 3 and then b is assigned to a, it looks like this:

Once we modify b by adding 9, b now refers to a new value, while a is still bound to the original value.

Notice that 3 still exists, unmodified. Now, a second value exists, and the name b is now bound to it, while a still refers to 3. This is very different than the list that we changed in-place. This is why we have write b = b + 9 instead of just b + 9. In order to change the value of b, we need to reassign the name a because we can't change the underlying value.

Both of these articles provide great explanations (full of pretty diagrams) of assignments and mutability:

• http://nedbatchelder.com/text/names.html
• http://mail.python.org/pipermail/tutor/2001-February/003787.html