[Tutor] subclassing list -- slicing doesn't preserve type

Brian van den Broek bvande at po-box.mcgill.ca
Tue Feb 22 20:26:32 CET 2005

Karl Pflästerer said unto the world upon 2005-02-22 07:53:
> On 22 Feb 2005, bvande at po-box.mcgill.ca wrote:
>>I'm trying to figure out how to subclass the list built-in.
>>.>>> class my_list(list):
>>         def __init__(self, sequence=None):
>>             list.__init__(self, sequence)
>>             self.spam = 1

<SNIP my examples of behaviour: notably, slicing doesn't preserve 
my_list type>

>>So, how can I get slicing to preserve the my_list type? And why does
>>the above class get so much for free, but not slicing?
> It gets also slicing for free but think about what happens when you call
> e.g. `append'; you *mutate an existing object in place* (i.e. a
> destructive operation).  So you change the already existing object which
> is an instance of  Mylist.  On the other hand with slicing you get a
> *new* object back; you never told Python that you also wanted that new
> object to be of type Mylist.
> You could do it e.g. like that:
> class Mylist (list):
>     def __init__(self, seq=None):
>         super(self.__class__, self).__init__(seq)
>     def __getslice__(self, start, stop):
>         return self.__class__(super(self.__class__, self).__getslice__(start, stop))
>     def __getitem__(self, key):
>         if isinstance(key, slice):
>             return self.__class__(super(self.__class__, self).__getitem__(key))
>         else:
>             return super(self.__class__, self).__getitem__(key) 
> For simple slices Python calls `list.__getslice__' for extended slices
> list.__getitem__ gets called.  So the above handles all cases.

<SNIP Karl's similar output, but with desired behaviour ;-) >

> You must also think about the other methods which return a newly
> allocated object.
>    Karl

Thanks Karl (and Kent in another response),

use of super is new to me; from a quick glance at the docs, I am glad 
to have been pointed at it. For the quick and dirty, I think I will go 
the __getslice__ way, deprecation be damned. But, learning how to do 
it the way you have above is high on the list of things to do.

Thanks and best,

Brian vdB

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