"""
Iterator Design Pattern
Intent: Lets you traverse elements of a collection without exposing its
underlying representation (list, stack, tree, etc.).
"""
from __future__ import annotations
from collections.abc import Iterable, Iterator
from typing import Any, List
"""
To create an iterator in Python, there are two abstract classes from the built-
in `collections` module - Iterable,Iterator. We need to implement the
`__iter__()` method in the iterated object (collection), and the `__next__ ()`
method in theiterator.
"""
class AlphabeticalOrderIterator(Iterator):
"""
Concrete Iterators implement various traversal algorithms. These classes
store the current traversal position at all times.
"""
"""
`_position` attribute stores the current traversal position. An iterator may
have a lot of other fields for storing iteration state, especially when it
is supposed to work with a particular kind of collection.
"""
_position: int = None
"""
This attribute indicates the traversal direction.
"""
_reverse: bool = False
def __init__(self, collection: WordsCollection, reverse: bool = False) -> None:
self._collection = collection
self._reverse = reverse
self._position = -1 if reverse else 0
def __next__(self):
"""
The __next__() method must return the next item in the sequence. On
reaching the end, and in subsequent calls, it must raise StopIteration.
"""
try:
value = self._collection[self._position]
self._position += -1 if self._reverse else 1
except IndexError:
raise StopIteration()
return value
class WordsCollection(Iterable):
"""
Concrete Collections provide one or several methods for retrieving fresh
iterator instances, compatible with the collection class.
"""
def __init__(self, collection: List[Any] = []) -> None:
self._collection = collection
def __iter__(self) -> AlphabeticalOrderIterator:
"""
The __iter__() method returns the iterator object itself, by default we
return the iterator in ascending order.
"""
return AlphabeticalOrderIterator(self._collection)
def get_reverse_iterator(self) -> AlphabeticalOrderIterator:
return AlphabeticalOrderIterator(self._collection, True)
def add_item(self, item: Any):
self._collection.append(item)
if __name__ == "__main__":
# The client code may or may not know about the Concrete Iterator or
# Collection classes, depending on the level of indirection you want to keep
# in your program.
collection = WordsCollection()
collection.add_item("First")
collection.add_item("Second")
collection.add_item("Third")
print("Straight traversal:")
print("\n".join(collection))
print("")
print("Reverse traversal:")
print("\n".join(collection.get_reverse_iterator()), end="")Straight traversal:
First
Second
Third
Reverse traversal:
Third
Second
First