Python中的动态自引用字典

Question

我曾见过some SO questions制作引用同一词典其他部分的字典，但它们似乎都是为了在第一次创建字典时进行静态返回。有办法做到这一点并保持活力吗？

就像这样：

# Lets just say we have a barn with lots of animals
# and the pigs always eat 2 chickens, so there has to always be 2 chickens per pig
def make_barn():
    barn = {'pig': 0, 'cow': 0, 'chicken': 2*barn['pig']}
    return barn

因此：

barn['pig'] = 3
print barn

将打印如下：

{'pig':3, 'chicken':6, 'cow':0}

我尝试使用lambda函数或类，但我尝试将其作为一个庞大的字典进行筛选，以便在multiprocessing模块中的进程之间传递，而该模块似乎不喜欢lambda或class结构。

编辑：

我尝试使用在多处理脚本中给出的答案，似乎它是不可挑选的：

import multiprocessing as mp
import random
from UserDict import UserDict

class Barn(UserDict):
    '''Barn automatically sets correct values for chickens.
    TBD: What to do if chickens get changed?'''
    def __init__(self, *args, **kw):
        UserDict.__init__(self, *args, **kw)
        pigs = self.data.get('pigs', None)
        if pigs is not None:
            self.data['chicken'] = pigs * 2

    def __setitem__(self, name, value):
        if name == 'pigs':
            self.data['chicken'] = value * 2
        self.data[name] = value

def count_animals(barn_record):
    for animal in random.randrange(0,10,1):
        barn_record['pigs'] = animal

# We need this to 'if __name__ == '__main__':' business to avoid having too many farms created continuously
# See here : https://stackoverflow.com/questions/1923706/multiprocessing-launching-too-many-instances-of-python-vm
if __name__ == '__main__':
    manager = mp.Manager()
    barn_record = manager.dict({'pigs':3,'cows':2})
    farm = mp.Process(target = count_animals, args=(manager,))
    farm.start()
    print barn_record

Answer 1

也许是这样的？用Python2.7进行测试。

from UserDict import UserDict

class Barn(UserDict):
    '''Barn automatically sets correct values for chickens.
    TBD: What to do if chickens get changed?'''
    def __init__(self, *args, **kw):
        UserDict.__init__(self, *args, **kw)
        pigs = self.data.get('pigs', None)
        if pigs is not None:
            self.data['chicken'] = pigs * 2

    def __setitem__(self, name, value):
        if name == 'pigs':
            self.data['chicken'] = value * 2
        self.data[name] = value

if __name__ == '__main__':
    b = Barn(cows=1, pigs=2)
    print b
    b['pigs'] = 3
    print b

运行它应该会产生如下内容：

{'cows': 1, 'chicken': 4, 'pigs': 2}
{'cows': 1, 'chicken': 6, 'pigs': 3}