在Python的自定义类上挂钩__dict__？

Question

I want to execute a method before anything is read from my class in a transparent way (so that the caller does not have to make the call before writing to the class instance). Hooking __getattr__ is easy, but I cannot figure out how to hook __dict__ since object does not have the __dict__ attribute.

super().__dict__ gives 'super' object has no attribute '__dict__'

Maybe there is something else I should do to accomplish this?

Answer 1

You don't want to use the class's __dict__ at all, in most cases (it's shared between all instances, remember), and you almost certainly never want the supertype's __dict__ , given that (as you have just discovered) it needn't necessarily have one.

>>> class One:
...     def __init__(self):
...         print(super().__dict__)
...
>>> one = One()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 3, in __init__
AttributeError: 'super' object has no attribute '__dict__'

As you report, object has no __dict__ of its own. That's because object instances, being built-in types, aren't allowed to acquire additional attributes so object is hard-coded to disallow it:

>>> object().someattr = "hello"
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'object' object has no attribute 'someattr'
>>>

When you create a subclass of object in your program, however, its instances can take arbitrary attributes, which are stored in the instance __dict__ .

>>> class Two:
...     def __init__(self):
...         print(self.__dict__)
...
>>> two = Two()
{}
>>> two.someattr = "hello"
>>> two.__dict__
{'someattr': 'hello'}
>>>

Note that the inheritance mechanism makes the Two class appear to have all of the attributes of object . The dir() function lets you look at an object's namespace as modified by inheritance:

>>> dir(Two)
['__class__', '__delattr__', '__dict__', '__dir__', '__doc__', '__eq__', 
'__format__', '__ge__', '__getattribute__', '__gt__', '__hash__', '__init__', 
'__le__', '__lt__', '__module__', '__ne__', '__new__', '__reduce__', 
'__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', 
'__subclasshook__', '__weakref__']

Picking a random attribute, we can verify that the methods present on Two are actually inherited from object :

>>> Two.__delattr__ is object.__delattr__
True
>>>