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

[Marco Wahl]

rzed <rz*****@gmail.comwrites:

To simplify take

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

and

t = T()

and then you get

>>print t.a

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]

rzed wrote:

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?

__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]

Colin J. Williams wrote:

rzed wrote:

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

and I want to examine the 'a' attributes.

>>>>print T.a

1

>>>>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?

__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:

>>t = T()
print repr(t)
newt = NewT()
print repr(newt)
T.a
t.a

--Scott David Daniels
sc***********@acm.org

[Bruno Desthuilliers]

Marco Wahl a écrit :
(snip)

The __new__ method should return the class.

s/class/instance/