Python 迭代器模式讲解和代码示例

from __future__ import annotations from collections.abc import Iterable, Iterator from typing import Any """ 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._sorted_items = None # Will be set on first __next__ call self._position = 0 def __next__(self) -> Any: """ Optimization: sorting happens only when the first items is actually requested. """ if self._sorted_items is None: self._sorted_items = sorted(self._collection._collection) if self._reverse: self._sorted_items = list(reversed(self._sorted_items)) """ The __next__() method must return the next item in the sequence. On reaching the end, and in subsequent calls, it must raise StopIteration. """ if self._position >= len(self._sorted_items): raise StopIteration() value = self._sorted_items[self._position] self._position += 1 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 = None) -> None: self._collection = collection or [] def __getitem__(self, index: int) -> Any: return self._collection[index] def __iter__(self) -> AlphabeticalOrderIterator: """ The __iter__() method returns the iterator object itself, by default we return the iterator in ascending order. """ return AlphabeticalOrderIterator(self) def get_reverse_iterator(self) -> AlphabeticalOrderIterator: return AlphabeticalOrderIterator(self, True) def add_item(self, item: Any) -> None: 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("B") collection.add_item("A") collection.add_item("C") print("Straight traversal:") print("\n".join(collection)) print("") print("Reverse traversal:") print("\n".join(collection.get_reverse_iterator()), end="") 

Straight traversal: A B C Reverse traversal: C B A