python - Python：创建后冻结字典键

Question

是否可以在创建后“冻结”python dict，以便无法向其中添加新键？只能更改现有的键值。

如果没有，您如何知道何时更改现有键值对以及何时添加新键值对？

Answer 1

也许是这样的：

class FreezableDict (dict):
    __frozen = False

    def freeze (self):
        self.__frozen = True

    def __setitem__ (self, key, value):
        if self.__frozen and key not in self:
            raise ValueError('Dictionary is frozen')
        super().__setitem__(key, value)

>>> x = FreezableDict({'foo': 'bar', 'baz': 'bla'})
>>> x
{'baz': 'bla', 'foo': 'bar'}
>>> x['asdf'] = 'fdsa'
>>> x
{'asdf': 'fdsa', 'baz': 'bla', 'foo': 'bar'}
>>> x.freeze()
>>> x['hello'] = 'world'
Traceback (most recent call last):
  File "<pyshell#20>", line 1, in <module>
    x['hello'] = 'world'
  File "<pyshell#13>", line 8, in __setitem__
    raise ValueError('Dictionary is frozen')
ValueError: Dictionary is frozen

请注意，您可能还想覆盖其他方法，包括__delitem__、update、setdefault、pop和popitem，因为它们都可以修改字典。

---

如果您有兴趣完全锁定字典，您可以使用types.MappingProxyType它为您的字典提供只读视图。一旦你创建了你的普通字典，你就可以创建一个它的映射代理，它根本没有任何分配/更新功能。然后，您还可以删除对原始字典的任何引用（映射将保留一个），以防止它被用于进一步更新它：

>>> x = {'foo': 'bar'}
>>> y = types.MappingProxyType(x)
>>> y
mappingproxy({'foo': 'bar'})
>>> x['baz'] = 'bla'
>>> y
mappingproxy({'baz': 'bla', 'foo': 'bar'})
>>> y['hello'] = 'world'
Traceback (most recent call last):
  File "<pyshell#55>", line 1, in <module>
    y['hello'] = 'world'
TypeError: 'mappingproxy' object does not support item assignment
>>> del x
>>> y
mappingproxy({'baz': 'bla', 'foo': 'bar'})

或者只是在一行中，没有对原始字典的引用：

>>> x = types.MappingProxyType({'foo': 'bar', 'baz': 'bla'})
>>> x
mappingproxy({'baz': 'bla', 'foo': 'bar'})
>>> x['hello'] = 'world'
Traceback (most recent call last):
  File "<pyshell#60>", line 1, in <module>
    x['hello'] = 'world'
TypeError: 'mappingproxy' object does not support item assignment

Answer 2

这对于“香草”字典是不可能的。您可能想要子类化collections.MutableMapping. . .

^{未经测试的代码如下}

class FrozenKeyDict(collections.MutableMapping):
    """Mapping which doesn't allow keys to be added/deleted.

    It does allow existing key/value pairs to be modified.
    """
    def __init__(self, *args, **kwargs):
        self._frozen = False
        self._dict = {}
        super(FrozenKeyDict, self).__init__(*args, **kwargs)
        self._frozen = True

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

    def __setitem__(self, key, value):
        if self._frozen and key not in self._dict:
            raise KeyError('must be one of %s' % list(self))
        self._dict[key] = value

    def __delitem__(self, key):
        # modify to suit your needs ...
        raise KeyError('Removing keys not supported')

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

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