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 ...
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