__new__ woes with list

[Arnaud Delobelle]

macaronikazoo <macaronikazoo at gmail.com> writes:

> i'm having a hell of a time getting this to work.  basically I want to
> be able to instantiate an object using either a list, or a string, but
> the class inherits from list.
>
> if the class is instantiated with a string, then run a method over it
> to tokenize it in a meaningful way.
>
> so how come this doesn't work???  if I do this:
>
> a=TMP( 'some string' )
>
> it does nothing more than list('some string') and seems to be ignoring
> the custom __new__ method.
>
>
>
> def convertDataToList( data ): return [1,2,3]
> class TMP(list):
> 	def __new__( cls, data ):
> 		if isinstance(data, basestring):
> 			new = convertDataToList( data )
> 			return list.__new__( cls, new )
>
> 		if isinstance(data, list):
> 			return list.__new__( cls, data )

A list is mutable, its initialisation is done in __init__() not
__new__().  There was a recent post about this (in the last couple of
weeks).

-- 
Arnaud