What is the python attribute get and set order?

Question

Python provides us many possibilities on instance/class attribute, for example:

class A(object):
    def __init__(self):
        self.foo = "hello"

a = A()

There are many ways to access/change the value of self.foo:

1. direct access a.foo
2. inner dict a.__dict__['foo']
3. get and set a.__get__ and a.__set__,of course there two are pre-defined methods.
4. getattribute a.__getattribute__
5. __getattr__ and __setattr__
6. maybe more.

While reading source code, I always get lost of what's their ultimate access order? When I use a.foo, how do I know which method/attribute will get called actually?

Answer 1

bar = a.foo...

1. invokes a.__getattribute__('foo')
2. which in turn by default looks up a.__dict__['foo']
3. or invokes foo's .__get__() if defined on A.

The returned value would then be assigned to bar.

---

a.foo = bar...

1. invokes a.__getattribute__('foo')
2. which in turn by default looks up a.__dict__['foo']
3. or invokes foo's .__set__(bar) if defined on A.

Answer 2

I found out this great post that has a detailed explanation on object/class attribute lookup.

For object attribute lookup:

Assuming Class is the class and instance is an instance of Class, evaluating instance.foobar roughly equates to this:

• Call the type slot for Class.__getattribute__ (tp_getattro). The default does this:
  • Does Class.__dict__ have a foobar item that is a data descriptor ?
    • If yes, return the result of Class.__dict__['foobar'].__get__(instance, Class).
  • Does instance.__dict__ have a 'foobar' item in it?
    • If yes, return instance.__dict__['foobar'].
  • Does Class.__dict__ have a foobar item that is not a data descriptor [9]?
    • If yes, return the result of Class.__dict__['foobar'].__get__(instance, klass). [6]
• If the attribute still wasn't found, and there's a Class.__getattr__, call Class.__getattr__('foobar').

There is an illustrated image for this:

Please do check out the original blog if interested which gives a outstanding explanation on python class, attribute lookup, and metaclass.