Definitive documentation on newstyle classes? (WAS: Pickling and inheritance are making me hurt)

[Colin J. Williams]

Michele Simionato wrote:
>>It would be good to have a clear exposition of the pros and cons of
>>__new__ usage.
> 
> 
> It is there mostly because __init__ would not work on immutable types.
> OTOH, you probably do not want to use __new__ on mutable types
> (once I was caught by this trap, google for "overriding list.__new__"
> if you are interested).
>  
>    Michele Simionato
> 
Thanks.  The use of __new__ for immutable classes is described in:
          http://www.python.org/2.2.3/descrintro.html

This distinction hadn't registered with me before.

The Martelli bible doesn't bring this out.

It seems to me that __new__ should probably be deprecated for mutable 
classes.

Colin W.