python 手动迭代iter/next

with open('/etc/passwd') as f:
 while True:
 line = next(f, None)
 if line is None:
 break
 print(line, end='')

 items = [1, 2, 3]
 Get the iterator
 it = iter(items) # Invokes items.iter()
 Run the iterator
 next(it) # Invokes it.next()
 1
 next(it)
 2
 next(it)
 3
 next(it)
 Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 StopIteration

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)