The consequences of immutability

Python has two broad flavors of objects: mutable and immutable. A mutable object has an internal state that can be updated by using operators or method functions. An immutable object's state cannot be changed.

The canonical examples of immutable objects are the numbers. The number 2 must always have a single, immutable value midway between 1 and 3. We can't change the state of 2 to make it 3 without making a mockery of the idea of mathematical truth.

In Chapter 6, More Complex Data Types, we'll look at a number of mutable data structures. The most important three mutable collections are set, list, and dict. These objects can have items added, and removed; we can change the state of the object.

In addition to numbers being immutable, three other common structures are also immutable: str, bytes, and tuple. Because strings and bytes are immutable, the string manipulation methods will always create a new string object from one or more existing string objects.

This means we cannot mutate characters or substrings within a longer string. We might think we need to attempt something like this:

>>> word="vokalizers"
>>> word[2]= "c"

But this can't work because a string object is immutable. We always build new strings from the old string's parts. We do it like this:

>>> word= word[:2]+"c"+word[3:]

This works by extracting pieces of the original string and including new characters mixed with the old.