`namedtuple` 在 3.5.1 中有什么变化吗？

Question

在 Python 3.5.0:

>>> from collections import namedtuple
>>> cluster = namedtuple('Cluster', ['a', 'b'])
>>> c = cluster(a=4, b=9)
>>> c
Cluster(a=4, b=9)
>>> vars(c)
OrderedDict([('a', 4), ('b', 9)])

在 Python 3.5.1:

>>> from collections import namedtuple
>>> cluster = namedtuple('Cluster', ['a', 'b'])
>>> c = cluster(a=4, b=9)
>>> c
Cluster(a=4, b=9)
>>> vars(c)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: vars() argument must have __dict__ attribute

似乎 namedtuple 发生了变化（或者可能是 vars()？）。

这是故意的吗？难道我们不应该再使用这种模式将命名元组转换为字典吗？

Answer 1

来自docs

Named tuple instances do not have per-instance dictionaries, so they are lightweight and require no more memory than regular tuples.

docs（和help(namedtuple)）说要使用c._asdict()转换成字典。

Answer 2

根据 Python bug #24931:

[__dict__] disappeared because it was fundamentally broken in Python 3, so it had to be removed. Providing __dict__ broke subclassing and produced odd behaviors.

Revision that made the change

具体来说，没有定义 __slots__ 的子类会表现得很奇怪：

>>> Cluster = namedtuple('Cluster', 'x y')
>>> class Cluster2(Cluster):
    pass
>>> vars(Cluster(1,2))
OrderedDict([('x', 1), ('y', 2)])
>>> vars(Cluster2(1,2))
{}

使用._asdict().

Answer 3

__dict__ 作为 @property 实现并已被删除；你可以在源代码中看到变化：

3.5.0:

def __repr__(self):
    'Return a nicely formatted representation string'
    return self.__class__.__name__ + '({repr_fmt})' % self

@property
def __dict__(self):
    'A new OrderedDict mapping field names to their values'
    return OrderedDict(zip(self._fields, self))

def _asdict(self):
    'Return a new OrderedDict which maps field names to their values.'
    return self.__dict__

def __getnewargs__(self):
    'Return self as a plain tuple.  Used by copy and pickle.'
    return tuple(self)

def __getstate__(self):
    'Exclude the OrderedDict from pickling'
    return None

3.5.1:

def __repr__(self):
    'Return a nicely formatted representation string'
    return self.__class__.__name__ + '({repr_fmt})' % self

def _asdict(self):
    'Return a new OrderedDict which maps field names to their values.'
    return OrderedDict(zip(self._fields, self))

def __getnewargs__(self):
    'Return self as a plain tuple.  Used by copy and pickle.'
    return tuple(self)

`namedtuple` 在 3.5.1 中有什么变化吗？

Did something about `namedtuple` change in 3.5.1?

python

typeerror

python-3.5

namedtuple