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3.4. Classes in this module

When considering the class structure of the application, two classes are obvious:

However, we need a third class to represent the iterators that are returned by the .find() method in the SkipList class, and by that class's special .__iter__() method.

So we define a third class, _SkipListIterator, that defines a .next() method. This object keeps track of its position in the list by pointing to a _SkipItem.

How does this iterator know when it has reached the end of the skip list? It can't compare the _SkipItem to .__terminator, which is private to the SkipList class. Even if we made that attribute public, a _SkipItem object doesn't contain a reference to its containing SkipList.

The solution is to stipulate that the .__terminator object's forward links all point to itself. That way, a _SkipListIterator instance can detect when it is pointing at the terminator.

There is one more complication with this technique. What happens if someone has a copy of an iterator that points to a _SkipItem that has been deleted from its containing SkipList?

In order that the _SkipItem object correctly detects this erroneous situation, it must know when it represents a deleted element.

Therefore, we stipulate that in a _SkipItem instance the level-0 forward link must be set to None when it is deleted. That way, the .next() method will know to signal an error and not attempt to chase its forward link.