Python Mutable vs Immutable

To get started, it’s important to understand that every object in Python has an ID (or identity), a type, and a value, as shown in the following snippet:

age = 42
print(id(age))    # id
print(type(age))  # type
print(age)        # value
[Out:]
10966208
<class ‘int’>
42

Once created, the ID of an object never changes. It is a unique identifier for it, and it is used behind the scenes by Python to retrieve the object when we want to use it.

The type also never changes. The type tells what operations are supported by the object and the possible values that can be assigned to it.

The value can either change or not. If it can, the object is said to be mutable, while when it cannot, the object is said to be immutable.

Let’s take a look at an example:

age = 42
print(id(age))
print(type(age))
print(age)age = 43
print(age)
print(id(age))
[Out:]
10966208
<class ‘int’>
42
43
10966240

Has the value of age changed? Well, no. 42 is an integer number, of the type int, which is immutable. So, what happened is really that on the first line, age is a name that is set to point to an int object, whose value is 42.

When we type age = 43, what happens is that another object is created, of the type int and value 43 (also, the id will be different), and the name age is set to point to it. So, we didn’t change that 42 to 43. We actually just pointed age to a different location.

As you can see from printing id(age) before and after the second object named age was created, they are different.

Now, let’s see the same example using a mutable object.

x = [1, 2, 3]
print(x)
print(id(x))x.pop()
print(x)
print(id(x))
[Out:]
[1, 2, 3]
139912816421064
[1, 2]
139912816421064

For this example, we created a list named m that contains 3 integers, 1, 2, and 3. After we change m by “popping” off the last value 3, the ID of m stays the same!

So, objects of type int are immutable and objects of type list are mutable. Now let’s discuss other immutable and mutable data types!