python - 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:

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)

这段代码在没有 __len__ 方法的情况下运行良好。删除这三行并运行上面的代码输出:

  first
  second
  third

但是，如果存在 __len__ 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

请注意输出中存在 first。 print_list() 执行一次，但在递归之前隐式调用了 __len__() 方法。什么在调用该方法？

我在 python 3.3.1 和 2.7.3 中看到了相同的行为

Answer 1

您在 bool 上下文中使用此处:

while here:

这将使用 __len__ 来查看它是否是一个空容器(例如是 false-y)，参见 Truth Value Testing :

Any object can be tested for truth value, for use in an if or while condition or as operand of the Boolean operations below. The following values are considered false:

[...]

• instances of user-defined classes, if the class defines a __nonzero__() or __len__() method, when that method returns the integer zero or bool value False.

对于您的用例，请改用 is not None:

while here is not None: