python - 'self.__class__.__missing__'是什么意思

Question

在 pinax Userdict.py 中:

def __getitem__(self, key):
        if key in self.data:
            return self.data[key]
        if hasattr(self.__class__, "__missing__"):
            return self.__class__.__missing__(self, key)

为什么它在 self.__class__.__missing__ 上执行此操作。

谢谢

Answer 1

UserDict.py 呈现 here模拟内置 dict紧密地，例如:

>>> class m(dict):
...   def __missing__(self, key): return key + key
... 
>>> a=m()
>>> a['ciao']
'ciaociao'

就像你可以覆盖特殊方法一样__missing__在子类化内置 dict 时处理丢失的键，所以当你将 UserDict 子类化时你可以覆盖它吗？ .

dict 的官方 Python 文档是 here ，他们确实说:

New in version 2.5: If a subclass of dict defines a method __missing__(), if the key key is not present, the d[key] operation calls that method with the key key as argument. The d[key] operation then returns or raises whatever is returned or raised by the __missing__(key) call if the key is not present. No other operations or methods invoke __missing__(). If __missing__() is not defined, KeyError is raised. __missing__() must be a method; it cannot be an instance variable. For an example, see collections.defaultdict.