python - 用于将其他类的项目作为属性访问的代理类（__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。我想要一个代理类，它将使用属性来访问该字典。我看过很多关于覆盖的帖子__setitem__，__getitem__但我似乎无法做到这一点。

self.mapper天真的方法（如下）由于调用而导致无限递归LabelMapperProxy.__getattr__，这反过来又需要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？也许标准库中有这样的代理？

Answer 1

您正在尝试在您的实例上设置一个新属性：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__getattr__会被查询（所有缺失的属性都会调用它）。并__getattr__尝试访问self.mapper....

解决方法是mapper直接设置self.__dict__：

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

或者，仅将原始基类__setattr__用于mapper属性：

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

Answer 2

这是陷阱：

class LabelMapperProxy(object):
   def __init__(self, mapper):
       # This will not assign this object's attribute 
       # since __setattr__ is overriden.
       # Instead, it will do self.mapper['mapper'] = mapper
       self.mapper=mapper

   def __getattr__(self, key): 
       # And this won't find `self.mapper` and resort to __getattr__
       # (which is itself)
       return self.mapper[key]  

   def __setattr__(self, key, val): 
       self.mapper[key]=val

   def __delattr__(self, key): 
       del self.mapper[key]