为什么在自定义迭代器上隐式调用__len __()

Question

我正在编写一个简单的链表实现,如下所示:

class Node(object):
    def __init__(self, value):
        self.value = value
        self._next = None

    def __iter__(self):
        here = self
        while here:
            yield here
            here = here._next

    def __len__(self):
        print("Calling __len__ on: {}".format(self))
        return sum(1 for _ in self)

    def append_to_tail(self, value):
        if self._next is None:
            self._next = Node(value)
        else:
            self._next.append_to_tail(value)

def print_list(ll):
    print(ll.value)
    if ll._next:
        print_list(ll._next)

my_list = Node('a')
my_list.append_to_tail('b')
my_list.append_to_tail('c')

print_list(my_list)

没有__len__方法,此代码运行正常.删除这三行并运行上面的代码输出:

  first
  second
  third

但是,如果__len__存在该方法,则结果为:

    first
    Calling __len__ on: <__main__.Node object at 0x108804dd0>
    Calling __len__ on: <__main__.Node object at 0x108804dd0>
    <snip>
    Calling __len__ on: <__main__.Node object at 0x108804dd0>
    Calling __len__ on: <__main__.Node object at 0x108804dd0>
    Traceback (most recent call last):
      File "len_example.py", line 31, in <module>
        print_list(my_list)
      File "len_example.py", line 24, in print_list
        if ll._next:
      File "len_example.py", line 14, in __len__
        return sum(1 for _ in self)
      File "len_example.py", line 14, in <genexpr>
        return sum(1 for _ in self)
      File "len_example.py", line 8, in __iter__
        while here:
      File "len_example.py", line 14, in __len__
        return sum(1 for _ in self)
      File "len_example.py", line 14, in <genexpr>
        return sum(1 for _ in self)
    <snip>
      File "len_example.py", line 8, in __iter__
        while here:
      File "len_example.py", line 13, in __len__
        print("Calling __len__ on: {}".format(self))
    RuntimeError: maximum recursion depth exceeded while calling a Python object

注意first输出中是否存在.print_list()执行一次,但__len__()在递归之前隐式调用该方法.什么叫这种方法？

我看到python 3.3.1和2.7.3的相同行为

Answer 1

您正在here布尔上下文中使用:

while here:

这将用于__len__查看它是否为空容器(例如是假y),请参阅真值测试:

任何对象都可以用于真值进行测试,用于在使用if或while病症或如下面的布尔运算的操作数.以下值被视为false:

[...]

• 用户定义的类的实例,如果类定义了一个__nonzero__()或__len__()方法,则该方法返回整数零或bool值False.

对于您的用例,请is not None改用:

while here is not None: