Difference between __getattribute__ and obj.__dict__['x'] in python?

Question

I understand that in python, whenever you access a class/instance variable, it will call __getattribute__ method to get the result. However I can also use obj.__dict__['x'] directly, and get what I want.

I am a little confused about what is the difference? Also when I use getattr(obj, name), is it calling __getattribute__ or obj.__dict__[name] internally?

Thanks in advance.

Answer 1

__getattribute__() method is for lower level attribute processing.

Default implementation tries to find the name in the internal __dict__ (or __slots__). If the attribute is not found, it calls __getattr__().

UPDATE (as in the comment):

They are different ways for finding attributes in the Python data model. They are internal methods designed to fallback properly in any possible situation. A clue: "The machinery is in object.__getattribute__() which transforms b.x into type(b).__dict__['x'].__get__(b, type(b))." from docs.python.org/3/howto/descriptor.html

Answer 2

Attributes in __dict__ are only a subset of all attributes that an object has.

Consider this class:

class C:
    ac = "+AC+" 
    def __init__(self):
        self.ab = "+AB+" 

    def show(self):
        pass

An instance ic = C() of this class will have attributes 'ab', 'ac' and 'show' (and few others). The __gettattribute__ will find them all, but only the 'ab' is stored in the ic.__dict__. The other two can be found in C.__dict__.