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.
4

So far, it's as I expect, but:
Traceback (most recent call last):
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?

 

[Marco Wahl]

To simplify take

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

and

t = T()

and then you get

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

While T.a is 1.

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?

I don't know.

How would it be used
(or, I guess, how should the __new__() method be used)? Any hints?

The __new__ method should return the class. In your
case return is None. Further the parametername for the
__new__ method should be better cls to have a
distinction to the usual self for instances.

See http://www.python.org/doc/2.4.4/ref/customization.html


Best wishes

 

[Colin J. Williams]

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.

4

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

Traceback (most recent call last):

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?

__new__ should return something. Since there is no return statement,
None is returned.

You might try something like:

class T(object):
def __new__(cls):
cls= object.__new__(cls)
cls.a= 1
return cls
t= T()
print t.a

Colin W.

 

[Scott David Daniels]

__new__ should return something. Since there is no return statement,
None is returned.

You might try something like:

class T(object):
def __new__(cls):
cls= object.__new__(cls)
cls.a= 1
return cls
t= T()
print t.a

Or, to use a bit more revealing names:
class NewT(object):
def __new__(class_):
instance = object.__new__(class_)
instance.a = 1
return instance

You might have figured more of this out with:

--Scott David Daniels
(e-mail address removed)

 

[Bruno Desthuilliers]

Marco Wahl a écrit :
(snip)

The __new__ method should return the class.

s/class/instance/