The difference between using __init__ and setting a class variable

Question

I am trying to learn descriptors, and I am confused by the behavior of objects - in the two examples below, as I understand __init__, they should work the same way. Can someone disconnect me or point me to a resource that explains this?

import math
class poweroftwo(object):
    """any time this is set with an int, turns it value to a tuple of the int
    and the int^2"""
    def __init__(self, value=None, name="var"):
        self.val = (value, math.pow(value, 2))
        self.name = name

    def __set__(self, obj, val):
        print "SET"
        self.val = (val, math.pow(val, 2))
    def __get__(self, obj, objecttype):
        print "GET"
        return self.val

class powoftwotest(object):
    def __init__(self, value):
        self.x = poweroftwo(value)

class powoftwotest_two(object):
    x = poweroftwo(10)


>>> a = powoftwotest_two()
>>> b = powoftwotest(10)
>>> a.x == b.x
>>> GET
>>> False #Why not true? shouldn't both a.x and b.x be instances of poweroftwo with the same values?

Answer 1

First, please name all the classes with the names LeadingUpperCase.

>>> a.x
GET
(10, 100.0)
>>> b.x
<__main__.poweroftwo object at 0x00C57D10>
>>> type(a.x)
GET
<type 'tuple'>
>>> type(b.x)
<class '__main__.poweroftwo'>

a.x- This is instance-level access that supports descriptors. This is what is meant in section 3.4.2.2, "(the so-called descriptor class) appears in the class dictionary of another new-style class." An instance must be available for the dictionary class to use the __get__and methods __set__.

b.x - , .