• 如果需要对象可以作为 for in关键字或者join函数的参数，需要这个对线是一个可迭代对象即这个对象是iterable的
• python中可迭代对象（如list, str, dict, generator）是实现了 iter方法的对象，iter方法返回一个iterator（即实现了next方法的对象）

      """
      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="")