4.4 使用生成器函数实现迭代器协议

以实现一个深度优先方式遍历树形节点的生成器为例：

class Node:
    def __init__(self, value):
        self._value = value
        self._children = []
    def __repr__(self):
        return f"Node({self._value})"
    def add_child(self, node):
        self._children.append(node)
    def __iter__(self):
        return iter(self._children)
    def depth_first(self):
        yield self
        for node in self:
            yield from node.depth_first()
# 定义根节点
root = Node(0)
# 定义子节点
child1 = Node(1)
child2 = Node(2)
root.add_child(child1)
root.add_child(child2)
# 添加孙节点
child1.add_child(Node(3))
child2.add_child(Node(4))
child2.add_child(Node(5))
for node in root.depth_first():
    print(node)  # Node(0) Node(1) Node(3) Node(2) Node(4) Node(5)