Why no list heritable type?

Dave Benjamin ramen at lackingtalent.com
Wed Dec 22 23:00:42 CET 2004

Mike Meyer wrote:
> "Robert Brewer" <fumanchu at amor.org> writes:
>>>Why aren't built 
>>>in lists and dictionaries real heritable types that can save 
>>>this kind of patchwork?
>>They are since Python 2.2 (IIRC):
> And before Python 2.2 there was the UserList class in the standard
> library. Which is still there in 2.4. Shouldn't it be depreciated by
> this point?

Please keep in mind that the latest stable version of Jython is 2.1, 
which does not support subclassing "list", and "dict" does not exist at all:

Jython 2.1 on java1.4.2_01 (JIT: null)
Type "copyright", "credits" or "license" for more information.
.>>> class C(list): pass
Traceback (innermost last):
   File "<console>", line 1, in ?
TypeError: base is not a class object: instance of 
.>>> class D(dict): pass
Traceback (innermost last):
   File "<console>", line 1, in ?
NameError: dict

I'm sure there are workarounds that will make the above code work in 
both CPython and Jython, but until there is a stable release of Jython 
that natively supports subclassing "list" and "dict", I'd appreciate it 
if we didn't deprecate (or depreciate =) UserList/Dict just yet. I 
maintain several modules that are portable to both implementations.


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