python - 为什么重写 __contains__ 会破坏 OrderedDict.keys？

Question

我正在对 OrderedDict(Cpython，2.7.3)进行子类化以表示数据文件。 __getitem__ 从数据文件中提取一个字段并将其设置在当前实例上，类似于我在下面发布的代码。现在我想覆盖 __contains__ 以返回 True 如果该字段在字典中或在磁盘上的文件中，因为它可以通过任何一种方式读取。但是，这似乎破坏了 OrderedDict 检查其键的能力。

from collections import OrderedDict

dictclass = OrderedDict

class Foo(dictclass):
    def __getitem__(self,key):
        try:
            return dictclass.__getitem__(self,key)
        except KeyError:
            pass

        data = key*2
        self[key] = data
        return data

    def __contains__(self,whatever):
        return dictclass.__contains__(self,whatever) or 'bar' in whatever

a = Foo()
print a['bar']
print a.keys()

如果你运行上面的代码，你会得到这个输出:

barbar
[]

请注意，如果您在上面的代码中更改 dictclass = dict，它似乎仍然有效(给出以下输出)。

barbar
['bar']

我是不是做错了什么？

Answer 1

当 Foo.__contains__ 未定义时:

a['bar']

调用 Foo.__getitem__，执行

    self[key] = data

这调用了 OrderedDict.__setitem__，它是这样定义的:

def __setitem__(self, key, value, PREV=0, NEXT=1, dict_setitem=dict.__setitem__):
    'od.__setitem__(i, y) <==> od[i]=y'
    # Setting a new item creates a new link at the end of the linked list,
    # and the inherited dictionary is updated with the new key/value pair.
    if key not in self:
        root = self.__root
        last = root[PREV]
        last[NEXT] = root[PREV] = self.__map[key] = [last, root, key]
    dict_setitem(self, key, value)

因为 Foo.__contains__ 没有定义，

    if key not in self:

为真。所以 key 被正确地添加到 self.__root 和 self.__map。

当Foo.__contains__被定义时，

    if key not in self:

如果为假。因此 key 未正确添加到 self.__root 和 self.__map。Foo.__contains__ 有效愚弄 OrderedDict.__setitem__ 认为 'bar' 键已经被添​​加。

---

我发现使用以下代码(在 __setitem__ 和 __iter__ 中添加打印语句)很有帮助:

from collections import OrderedDict

dictclass = OrderedDict

class Foo(dictclass):
    def __getitem__(self,key):
        try:
            return dictclass.__getitem__(self,key)
        except KeyError:
            pass

        data = key*2
        self[key] = data
        return data

    def __contains__(self,whatever):
        print('contains: {}'.format(whatever))
        return dictclass.__contains__(self,whatever) or 'bar' in whatever

    def __setitem__(self, key, value, PREV=0, NEXT=1, dict_setitem=dict.__setitem__):
        'od.__setitem__(i, y) <==> od[i]=y'
        # Setting a new item creates a new link at the end of the linked list,
        # and the inherited dictionary is updated with the new key/value pair.
        print('key not in self: {}'.format(key not in self))
        if key not in self:
            root = self._OrderedDict__root
            last = root[PREV]
            last[NEXT] = root[PREV] = self._OrderedDict__map[key] = [last, root, key]
        dict_setitem(self, key, value)

    def __iter__(self):
        'od.__iter__() <==> iter(od)'
        # Traverse the linked list in order.
        NEXT, KEY = 1, 2

        root = self._OrderedDict__root
        curr = root[NEXT]
        print('curr: {}'.format(curr))
        print('root: {}'.format(root)) 
        print('curr is not root: {}'.format(curr is not root))

        while curr is not root:
            yield curr[KEY]
            curr = curr[NEXT]

a = Foo()
print a['bar']
# barbar

print a.keys()
# ['bar']

---

请注意，您可以通过使 Foo 成为 collections.MutableMapping 的子类并将其大部分行为委托(delegate)给 OrderedDict 属性来避免此问题:

import collections
dictclass = collections.OrderedDict

class Foo(collections.MutableMapping):
    def __init__(self, *args, **kwargs):
        self._data = dictclass(*args, **kwargs)
    def __setitem__(self, key, value):
        self._data[key] = value
    def __delitem__(self, key):
        del self._data[key]
    def __iter__(self):
        return iter(self._data)
    def __len__(self):
        return len(self._data)

    def __getitem__(self,key):
        try:
            return self._data[key]
        except KeyError:
            pass

        data = key*2
        self[key] = data
        return data

    def __contains__(self,whatever):
        return dictclass.__contains__(self,whatever) or 'bar' in whatever

产生

a = Foo()
print a['bar']
# barbar

print a.keys()
# ['bar']

即使定义了 __contains__。