4.4 实现迭代器协议
目前为止,在一个对象上实现迭代最简单的方式是使用一个生成器函数。在4.2小节中,使用Node类来表示树形数据结构。你可能想实现一个以深度优先方式遍历树形节点的生成器。下面是代码示例:
Python的迭代协议要求一个 iter()
方法返回一个特殊的迭代器对象,这个迭代器对象实现了 方法并通过 StopIteration
异常标识迭代的完成。但是,实现这些通常会比较繁琐。下面我们演示下这种方式,如何使用一个关联迭代器类重新实现 方法:
- class Node2:
- def __init__(self, value):
- self._value = value
- self._children = []
- def __repr__(self):
- return 'Node({!r})'.format(self._value)
- def add_child(self, node):
- self._children.append(node)
- def __iter__(self):
- return iter(self._children)
- def depth_first(self):
- return DepthFirstIterator(self)
- class DepthFirstIterator(object):
- '''
- Depth-first traversal
- '''
- def __init__(self, start_node):
- self._node = start_node
- self._children_iter = None
- self._child_iter = None
- def __iter__(self):
- return self
- def __next__(self):
- # Return myself if just started; create an iterator for children
- if self._children_iter is None:
- self._children_iter = iter(self._node)
- return self._node
- # If processing a child, return its next item
- elif self._child_iter:
- try:
- nextchild = next(self._child_iter)
- return nextchild
- except StopIteration:
- self._child_iter = None
- return next(self)
- # Advance to the next child and start its iteration
- else:
- self._child_iter = next(self._children_iter).depth_first()
原文: