Iterator Design Pattern in Python...

Iterator design pattern is a behavioral design pattern that lets you access the element of a complex data structure - a collection class - without the knowledge of the internal structure of the elements it is accessing.

To create an iterator in Python, there are two abstract classes from the built-in `collections` module - Iterable, and Iterator. We need to implement the

`__iter__()` method in the iterated object (collection), and the `__next__ ()` method in the iterator.

The concrete class that implements the Iterator class provides various means of different ways of iteration through the collection class.

In our example, AlphabeticalOrderIterator is such a class that has provided forward and reverse ways of iteration. In this example, the WordsCollection class represents a collection of words and implements the Iterable interface.

The source code is as follows:

from collections.abc import Iterator, Iterable
from typing import Any, List

class WordsCollection(Iterable):
    def __init__(self, collection: List[Any]=[]):
        self._collection = collection

    def __iter__(self):
        return AlphabeticalOrderIterator(self._collection)

    def get_reverse_iterator(self):
        return AlphabeticalOrderIterator(self._collection, True)

    def add_item(self, item:Any):
        self._collection.append(item)

class AlphabeticalOrderIterator(Iterator):
    _position = None
    _reverse = False

    def __init__(self, collection : WordsCollection, reverse: bool = False):
        self._collection = collection
        self._reverse = reverse
        self._position = -1 if self._reverse else 0

    def __next__(self):
        try:
            value = self._collection[self._position]
            self._position += -1 if self._reverse else 1
        except IndexError:
            raise StopIteration()

        return value


# Press the green button in the gutter to run the script.
if __name__ == '__main__':
    wordsCollection = WordsCollection()
    wordsCollection.add_item("Som")
    wordsCollection.add_item("Reema")
    wordsCollection.add_item("Ridit")

    print("Forward traversal:")
    print("\n".join(wordsCollection))
    print("")

    print("Reverse traversal:")
    print("\n".join(wordsCollection.get_reverse_iterator()), end="")

If we run the above program the output will be as follows:

Forward traversal:

Som

Reema

Ridit

Reverse traversal:

Ridit

Reema

Som