Shaun Shaun - 1 month ago 11
Python Question

Usefulness of def __init__(self)?

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?

Answer

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.

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