Class and instance question

[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
scott.daniels at acm.org