# 4.4 实现迭代器协议¶

## 解决方案¶

```class Node:
def __init__(self, value):
self._value = value
self._children = []

def __repr__(self):
return 'Node({!r})'.format(self._value)

self._children.append(node)

def __iter__(self):
return iter(self._children)

def depth_first(self):
yield self
for c in self:
yield from c.depth_first()

# Example
if __name__ == '__main__':
root = Node(0)
child1 = Node(1)
child2 = Node(2)

for ch in root.depth_first():
print(ch)
# Outputs Node(0), Node(1), Node(3), Node(4), Node(2), Node(5)
```

## 讨论¶

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 `__next__()` 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 `depth_first()` 方法：

```class Node2:
def __init__(self, value):
self._value = value
self._children = []

def __repr__(self):
return 'Node({!r})'.format(self._value)

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()
return next(self)
```

`DepthFirstIterator` 类和上面使用生成器的版本工作原理类似， 但是它写起来很繁琐，因为迭代器必须在迭代处理过程中维护大量的状态信息。 坦白来讲，没人愿意写这么晦涩的代码。将你的迭代器定义为一个生成器后一切迎刃而解。