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如果字段在字典中或磁盘上的文件中，因为它可以以任何方式读取。但是，这似乎打破了OrderedDict检查密钥的能力。

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()

如果您运行上面的代码，您将获得此输出：

barbar
[]

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

barbar
['bar']

我做错了什么？

Answer 1

未定义Foo.__contains__时：

a['bar']

调用Foo.__getitem__，执行

    self[key] = data

这会调用OrderedDict.__setitem__，这是以这种方式定义的：

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)

由于未定义Foo.__contains__，

    if key not in self:

是真的。因此，密钥已正确添加到self.__root和self.__map。

定义Foo.__contains__时，

    if key not in self:

如果错误。因此，密钥未正确添加到self.__root和self.__map。 Foo.__contains__有效的傻瓜OrderedDict.__setitem__认为已经添加了'bar'密钥。

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，您可以通过将Foo作为collections.MutableMapping的子类并将其大多数行为委托给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__。

Answer 2

or 'bar' in whatever会破坏您的代码。如果您将其删除，它将与您提及的更改dictclass = dict一样工作。

__setitem__的{{1}}实现是：

OrderedDict

因此，对于def __setitem__(self, key, value, 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[0] last[1] = root[0] = self.__map[key] = [last, root, key] return dict_setitem(self, key, value)，条件应该为False，但即使在插入任何项目之前它也是True。因此，密钥未添加到self["bar"] = "barbar"中使用的self.__root：

OrderedDict.__iter__

由于检索值的代码使用此迭代器而def __iter__(self): 'od.__iter__() <==> iter(od)' # Traverse the linked list in order. root = self.__root curr = root[1] # start at the first node while curr is not root: yield curr[2] # yield the curr[KEY] curr = curr[1] # move to next node不包含self.__root，因此无法在值中返回此具体键。

为什么重写contains会破坏OrderedDict.keys？

2 个答案:

为什么重写__contains__会破坏OrderedDict.keys？

2 个答案:

为什么重写contains会破坏OrderedDict.keys？