用于访问其他类的项作为属性的代理类(__getitem__无限递归)

Question

我有一个类LabelMapper(一个boost::python类),它实现了字典协议.我想有一个代理类,它将使用属性来访问该dicionary.我已经看过许多重写的帖子__setitem__,__getitem__但我似乎无法做到正确.

天真的方法(下面)导致无限递归,因为self.mapper调用LabelMapperProxy.__getattr__,反过来需要self.mapper等等.

class LabelMapper(object):
   def __init__(self): self.map={}
   def __getitem__(self,key): return self.map[key]
   def __setitem__(self,key,val): self.map[key]=val
   def __delitem__(self,key): del self.map[key]

class LabelMapperProxy(object):
   def __init__(self,mapper): self.mapper=mapper
   def __getattr__(self,key): return self.mapper[key]
   def __setattr__(self,key,val): self.mapper[key]=val
   def __delattr__(self,key): del self.mapper[key]

lm=LabelMapper()
lm['foo']=123

# construct the proxy
lmp=LabelMapperProxy(mapper=lm)
print lmp.foo                    # !!! recursion
lmp.bar=456
print lmp.bar,lm['bar']

解决办法是什么？也许在标准库中预先煮好这样的代理？

Answer 1

您正尝试在实例上设置新属性Proxy:

class LabelMapperProxy(object):
    def __init__(self, mapper): self.mapper = mapper

这会触发a __setattr__,它会尝试访问不存在的self.mapper属性,因此会__getattr__被查询(对所有缺少的属性都会调用).并__getattr__尝试访问self.mapper....

解决方案是mapper直接设置self.__dict__:

class LabelMapperProxy(object):
    def __init__(self, mapper): self.__dict__['mapper'] = mapper

或者,__setattr__仅为mapper属性使用原始基类:

class LabelMapperProxy(object):
   def __init__(self, mapper): self.mapper = mapper

   def __setattr__(self, key, val):
       if key == 'mapper':
           return super(LabelMapperProxy, self).__setattr__(key, val)
       self.mapper[key] = val