Python- initialisation of built in variables/attributes

Question

When and how are the builtin attributes initialized? __doc__, __name__ (I guess I know about this one :) ), __class__, __setattr__ etc.

In my other question regarding docstrings, one of the answers mentioned that docstrings are just simple strings and I tried with ', " and """ and they all worked. But when I used a variable assigned the string value and put that variable in place of the docstring, it doesn't work. That's why I started wondering when does the __doc__ attribute get initialized?

EDIT: This is what I have tried on interpreter (yes, this is crazy and I am wierd :D)

doc_str = "Says Hello world"


class HelloWorld():
    def say():
        doc_str
        print("Hello world !")

h_w = HelloWorld()
h_w.say.__doc__


class AnotherHelloWorld():

    def __init__(self, doc_str="Says HELLO WORLD"):
        self.doc_str = doc_str

    def say(self):
        self.doc_str
        print("HELLO WORLD !")

a_h_w = AnotherHelloWorld("Scream... HELLO WORLD!")
a_h_w.say.__doc__


class YetAnotherHelloWorld():

    def __init__(self, doc_str="Still does't say HELLO WORLD :( "):
        self.doc_str = doc_str

    def say(self):
          "%s"%self.doc_str
          print("HELLO WORLD .. Again!")

Answer 1

It's different for each one. (After all, each one's special!) Some are the class attributes, some are instance attributes, some are inherited.

__doc__ is initialized when the class is created (you can also pass it in the dict argument to the type constructor). The special syntax only works for string literals, but if you need a variable docstring you can set it explicitly:

class SomeClass(object):
    __doc__ = "This is class #{0}.".format(1)

__name__ is also set when the class is created.

__class__ is set when an instance is created (i.e. in __new__).

__setattr__ and friends are inherited from object.