changing the List's behaviour?

[Peter Otten]

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