Python中的動態自助參考字典

Question

我看過someSOquestions關於製作引用同一字典的其他部分的字典，但它們似乎都是在字典第一次創建時用於靜態返回。 有沒有辦法做到這一點，保持動態？

像這樣：

# 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 

這樣的：

barn['pig'] = 3 
print barn 

將打印：

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

我嘗試使用lambda函數或類，但我想泡菜這是一個巨大的字典，用於在multiprocessing模塊中的進程和該模塊似乎不喜歡lambda或class結構。

編輯：

我試圖用一個多腳本給出了答案，看來，它不是picklable：

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

像這樣的事情吧？經過Python 2.7測試。

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 

運行應該產生這樣的：

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