changing the List's behaviour?

[Heather Coppersmith]

On Wed, 30 Jul 2003 17:36:12 +0200,
Peter Otten <__peter__ at web.de> wrote:

> Heather Coppersmith wrote:
>>> class DefaultList(list):
>>> def __init__(self, sequence=[], default=None):
>                 list.init(self, sequence)
>> 
>> That's asking for trouble.  That mutable default argument for
>> sequence is evaluated at class definition-time, and all instances
>> of DefaultList created without a sequence argument will end up
>> sharing one list.
>> 
>> Do this instead:
>> 
>> class DefaultList( list ):
>>     def __init__( self, sequence = None, default = None ):
>>          if sequence is None:
>>             sequence = [ ]
>> 
>>> list.__init__(self, sequence)

> I can see the trouble only if the sequence argument is used to initialize a
> member, e.g.

> def __init__(self, seq=[]):
>         self.seq = seq # bad, multiple instances may share one list

> However, in the DefaultList case, sequence is never changed.
> So I don't see what can go wrong. Am I overlooking something?

> - Peter

Oops, my mistake.  ;-)

Regards,
Heather

-- 
Heather Coppersmith
That's not right; that's not even wrong. -- Wolfgang Pauli