python - Usefulness of def __init__(self)?

Question

I am fairly new to python, and noticed these posts: Python __init__ and self what do they do? and Python Classes without using def __init__(self)

After playing around with it, however, I noticed that these two classes give apparently equivalent results-

class A(object):
    def __init__(self):
        self.x = 'Hello'

    def method_a(self, foo):
        print self.x + ' ' + foo

(from this question)

and

class B(object):
    x = 'Hello'
    def method_b(self,foo):
        print self.x + ' ' + foo

Is there any real difference between these two? Or, more generally, does __init__ change anything inherently about the attributes of a class? In the documentation it is mentioned that __init__ is called when the instance is created. Does this mean that x in class B is established before instantiation?

question from:https://stackoverflow.com/questions/23302018/usefulness-of-def-init-self

Answer 1

Yeah, check this out:

class A(object):
    def __init__(self):
        self.lst = []

class B(object):
    lst = []

and now try:

>>> x = B()
>>> y = B()
>>> x.lst.append(1)
>>> y.lst.append(2)
>>> x.lst
[1, 2]
>>> x.lst is y.lst
True

and this:

>>> x = A()
>>> y = A()
>>> x.lst.append(1)
>>> y.lst.append(2)
>>> x.lst
[1]
>>> x.lst is y.lst
False

Does this mean that x in class B is established before instantiation?

Yes, it's a class attribute (it is shared between instances). While in class A it's an instance attribute. It just happens that strings are immutable, thus there is no real difference in your scenario (except that class B uses less memory, because it defines only one string for all instances). But there is a huge one in my example.