Python23: cannot inherit from bool?

[Greg Chapman]

On 16 Jan 2003 11:13:07 +0100, martin at v.loewis.de (Martin v. Löwis) wrote:

>Dale Strickland-Clark <dale at riverhall.NOTHANKS.co.uk> writes:
>
>> >.... don't know the answer to that one, but is the non inheritance
>> >property available to ordinary python if yes we could create leaf
>> >classes.
>> 
>> At the risk of sounding dim: what's a leaf class?
>
>I think the OP meant this as a synonym for what others call "final"
>classes, i.e. classes that cannot be used as a base class. The OP asks
>whether it is possible to implement a Python class which gives an
>error when it is used as a base. I believe the answer to that question
>is "no".
>

Wouldn't something like this work?:

LeafClass = None  #initialized so that it exists when LeafClass itself
                  #is being created

class LeafMetaClass(type):
    def __new__(metaclass, name, bases, attrs):
        if LeafClass is not None:
            for base in bases:
                if hasattr(base, '_leafclass_'):
                    raise TypeError('cannot inherit from '+base.__name__)
            attrs['_leafclass_'] = True
        return super(LeafMetaClass, metaclass).__new__(metaclass, name,
                                                       bases, attrs)

class LeafClass(object):
    '''Can serve as a mix-in base class to declare that it is illegal to
       inherit from a class'''
    __metaclass__ = LeafMetaClass

---
Greg Chapman