Python杂项

• iterator和iterable
  • 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="")