iterator.py

#!/usr/bin/env python
# encoding: utf-8
"""
迭代器模式

提供一种方法顺序地访问一个聚合对象中的各个元素, 而又不暴露改对象的内部表示

- 无差别遍历聚合对象
- 需要对聚集进行多种方式遍历时
- 分离了集合对象的遍历行为, 做到不暴露集合内部结构, 又可以让外部代码透明的访问集合内部的数据

用于在数据集合中按照顺序遍历集合, 让用户通过特定的接口巡访容器中的每一个元素而不用了解底层的实现
wiki: https://zh.wikipedia.org/wiki/%E8%BF%AD%E4%BB%A3%E5%99%A8%E6%A8%A1%E5%BC%8F
python:
    - http://nvie.com/posts/iterators-vs-generators/
    - https://docs.python.org/3.6/glossary.html#term-iterator
    - https://docs.python.org/3.6/library/collections.abc.html?highlight=iterable#collections.abc.Iterable
    - https://docs.python.org/3.6/library/collections.abc.html?highlight=iterator#collections.abc.Iterator
"""

# 不考虑默认的迭代器
from abc import ABCMeta, abstractmethod


class Iterator(metaclass=ABCMeta):
    """
    迭代抽象类

    定义迭代器抽象方法
    """

    @abstractmethod
    def __iter__(self):
        """ 
        should return self 
        this makes the class an Iterable
        """
        pass

    @abstractmethod
    def __next__(self):
        """
        Return the next item from the iterator. 
        When exhausted, raise StopIteration
        """
        pass


class ConcreteIterator(Iterator):
    def __init__(self, aggregate):
        self.__aggregate = aggregate
        self.index = 0


    def __next__(self):
        try:
            value=self.__aggregate[self.index]
        except IndexError:
            raise StopIteration()
        self.index+=1
        return value
    
        
    def __iter__(self):
        return self



# python 定义一个迭代器
import collections # noqa


class YRange(collections.Iterator):
    def __init__(self, n):
        self.i = 0
        self.n = n

    def __iter__(self):
        return self


    def __next__(self):
        if self.i < self.n:
            i = self.i
            self.i += 1
            return i
        else:
            raise StopIteration()


if __name__ == '__main__':
    aggregate ="a b c d e".split()

    print(issubclass(ConcreteIterator,collections.Iterator))

    it = ConcreteIterator(aggregate)
    i=iter(it)

    # equals to for j in i
    while True:
        try:
            print(next(i))
        except StopIteration:
            del i
            break
       
    y = YRange(3)
    print(list(y))

    y1 = YRange(5)
    print(iter(y1))