Class and instance question

[rzed]

I'm confused (not for the first time). 

I create these classes:

class T(object):
    def __new__(self):
        self.a = 1

class X(T):
    def __init__(self):
        self.a = 4

class Y:
    def __init__(self):
        self.a = 4

class Z(object):
    def __init__(self):
        self.a = 4


... and these instances:

t = T()
x = X()
y = Y()
z = Z()

and I want to examine the 'a' attributes.

>>> print T.a
1
>>> print y.a
4
>>> print z.a
4

So far, it's as I expect, but:

>>> print x.a
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'NoneType' object has no attribute 'a'

>>> print t.a
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'NoneType' object has no attribute 'a'

So what the heck is 'T'? It seems that I can't instantiate it or 
derive from it, so I guess it isn't a proper class. But it's 
something; it has an attribute. What is it? How would it be used 
(or, I guess, how should the __new__() method be used)? Any hints?

-- 
rzed