[Tutor] subclass question

[David Perlman]

If I make a subclass of a built-in class, like this:

class mylist(list):
     def __init__(self):
         list.__init__(self)

Then it is valid for me to do this:

 >>> x=mylist()
 >>> x.hello=3
 >>>

But I can't do this:

 >>> y=list()
 >>> y.hello=3
Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
AttributeError: 'list' object has no attribute 'hello'
 >>>

What is it that is special about built-in classes that prevents you  
from adding methods or, uh, whatever the generic term is for sub- 
variables?  Is there a way to make your own classes restricted like  
that?

OK thanks!


--
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