Python:创建后冻结dict键

Question

是否有可能在创建后"冻结"python dict,以便无法为其添加新密钥？只能更改现有的键值.

如果没有,您如何知道何时更改现有的键值对,以及何时添加新的键值对？

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,您可能要覆盖其他方法也包括__delitem__,update,setdefault,pop,和popitem,因为它们都可以修改的字典.

如果您对完全锁定字典感兴趣,可以使用types.MappingProxyType它为字典提供只读视图.一旦创建了普通字典,就可以创建它的映射代理,它只是没有任何赋值/更新功能.您也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,没有引用原始字典:

>>> 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