Python中的动态自引用词典

Question

我看到了一些关于制作引用同一词典其他部分的词典的SO问题，但无论何时首次创建词典，它们似乎都是静态返回。有没有办法做到这一点并保持活力？

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

这样：

barn['pig'] = 3
print barn

将打印此：

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

我尝试使用lambda函数或类，但我试图将其作为一个巨大的字典，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：

我尝试使用多处理脚本中给出的答案，但它似乎是不可选择的：

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

可能是这样的吗？使用Python 2.7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

运行应该会产生这样的结果：

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