__new__ woes with list

[macaronikazoo]

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 )

 

[Arnaud Delobelle]

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).

 

[macaronikazoo]

ok thansk - i will search again. i did try searching but didn't find
anything relevant...

 

[Arnaud Delobelle]

ok thansk - i will search again. i did try searching but didn't find
anything relevant...

Here's a link to the thread on google groups:

http://groups.google.com/group/comp.lang.python/browse_thread/thread/7aff4eabc6182858

Unfortunately two threads seem to be intertwined: scroll down to when
the thread is renamed "python bug when subclassing list?".

HTH