modifying __new__ of list subclass

[Steven Bethard]

Ken Schutte wrote:
> I want an int-derived class that is initilized to one greater than what 
> it's constructor is given:
> 
> class myint(int):
>   def __new__(cls, intIn):
>     newint = int(intIn+1)
>     return int.__new__(cls, newint)

Or simply:

class myint(int):
     def __new__(cls, int_in):
         return int.__new__(cls, int_in + 1)

> Now, lets say I want a list class that 
> creates a list of strings, but appends "_" to each element.  I try the 
> same thing:
> 
> class mylist(list):
>   def __new__(cls, listIn):
>     newlist = list()
>     for i in listIn:
>       newlist.append(str(i) + "_")
>     print "newlist: ", newlist
>     return list.__new__(cls, newlist)

The __new__ method is for immutable types.  So things like str and int 
do their initialization in __new__.  But for regular mutable types, you 
should do your initialization in __init__::

     class mylist(list):
         def __init__(self, list_in):
             for item in list_in:
                 self.append(str(item) + '_')

STeve