Question: Inheritance from a buil-in type

[Asun Friere]

Duncan Booth <duncan at NOSPAMrcp.co.uk> wrote in message news:<Xns9405A01F17467duncanrcpcouk at 127.0.0.1>...

> 
> There are two rules. Generally, immutable objects get their initial values 
> from the constructor (__new__) while mutable objects are constructed with a 
> default value (e.g. empty list or dict) and are then set by the initialiser 
> (__init__) method. A few types which you might expect to be immutable are 
> actually mutable (e.g. property).
>

What is the thinking behind that? I mean you /can/ pass initial values
to a mutable using __new__,  eg

     class MyList (list) :
         def __new__(cls, *args, **kwargs) :
             return list.__new__(cls, *args, **kwargs)

or __init__ to pass values to a newly created immutable, eg

     class MyTuple (tuple) :
         def __init__(self, *args, **kwargs) :
             return super(tuple, self).__init__(*args, **kwargs)

can't you?  What's the pitfall?