34

I have a class like:

class A:
    def __init__(self):
        self.data = {}

and at some moment I want to prohibit self.data fields modification.

I've read in PEP-416 rejection notice that there are a lot of ways to do it. So I'd like to find what they are.

I tried this:

a = A()
a.data = types.MappingProxyType(a.data)

That should work but first, its python3.3+ and second, when I do this "prohibition" multiple times I get this:

>>> a.data = types.MappingProxyType(a.data)
>>> a.data = types.MappingProxyType(a.data)
>>> a.data
mappingproxy(mappingproxy({}))

though it would be much better to get just mappingproxy({}) as I am going to "prohibit" a lot of times. Check of isinstance(MappingProxyType) is an option, but I think that other options can exist.

Thanks

4

5 回答 5

40

使用collections.Mapping例如

import collections

class DictWrapper(collections.Mapping):

    def __init__(self, data):
        self._data = data

    def __getitem__(self, key): 
        return self._data[key]

    def __len__(self):
        return len(self._data)

    def __iter__(self):
        return iter(self._data)
于 2015-02-11T10:58:59.777 回答
32

这是快速（浅）只读字典的完整实现：

class ReadOnlyDict(dict):
    def __readonly__(self, *args, **kwargs):
        raise RuntimeError("Cannot modify ReadOnlyDict")
    __setitem__ = __readonly__
    __delitem__ = __readonly__
    pop = __readonly__
    popitem = __readonly__
    clear = __readonly__
    update = __readonly__
    setdefault = __readonly__
    del __readonly__
于 2015-06-25T12:05:28.593 回答
10

很简单，你只需覆盖默认 dict 的方法！
这是一个例子：

class ReadOnlyDict(dict):

    __readonly = False

    def readonly(self, allow=1):
        """Allow or deny modifying dictionary"""
        self.__readonly = bool(allow)

    def __setitem__(self, key, value):

        if self.__readonly:
            raise TypeError, "__setitem__ is not supported"
        return dict.__setitem__(self, key, value)

    def __delitem__(self, key):

        if self.__readonly:
            raise TypeError, "__delitem__ is not supported"
        return dict.__delitem__(self, key)

顺便说一句，您还可以删除.pop,.update和其他您需要的方法。只是玩弄它。

于 2013-09-26T08:40:51.113 回答
2

最好的方法是这样派生UserDict：

from collections import UserDict

class MyReadOnlyDict(UserDict):
   def my_set(self, key, val, more_params):
       # do something special
       # custom logic etc
       self.data[key] = val

   def __setitem__(self, key, val):
       raise NotImplementedError('This dictionary cannot be updated')

   def __delitem__(self, key):
       raise NotImplementedError('This dictionary does not allow delete')

这种方法的优点是您仍然可以在您的类中拥有可以通过访问来更新字典的内部方法self.data。

于 2020-05-09T17:13:26.857 回答
1

怎么样？这是@mouad 答案的更新。

import json
from collections import OrderedDict
from collections.abc import Mapping


class ReadOnlyJsonObject(Mapping):
    def __init__(self, data, dumps_kw: dict=None, loads_kw: dict=None):
        if dumps_kw is None:
            dumps_kw = dict()
        if loads_kw is None:
            self._loads_kw = dict(object_pairs_hook=OrderedDict)
        else:
            self._loads_kw = loads_kw

        if isinstance(data, str):
            self._json_string = data
        else:
            self._json_string = json.dumps(data, **dumps_kw)

    @property
    def _data(self):
        return json.loads(self._json_string, **self._loads_kw)

    def __getitem__(self, key):
        return self._data[key]

    def __len__(self):
        return len(self._data)

    def __iter__(self):
        return iter(self._data)

    def __str__(self):
        return self._json_string

不过，不确定性能。我在一个真实的项目中使用这个https://github.com/patarapolw/AnkiTools/blob/master/AnkiTools/tools/defaults.py

于 2018-07-11T11:58:06.210 回答