subclassable types

[Jean-Michel Pichavant]

----- Original Message -----
> Dear all,
> I am wondering what the rules are that determine whether a built-in
> type is
> subclassable or not.
> As examples, why can you base your classes on int or set,
> but not on bool or range?
> Also: can you use introspection to find out whether a type is valid
> as a
> base type?
> Thanks for your help!
> 
> 
> --
> http://mail.python.org/mailman/listinfo/python-list
> 

Hi,

The BDFL explaining why you cannot subclass bool:

http://mail.python.org/pipermail/python-dev/2002-March/020822.html

"A subclass would only be useful when
it has instances, but the mere existance of an instance of a subclass
of bool would break the invariant that True and False are the only
instances of bool!  (An instance of a subclass of C is also an
instance of C.)  I think it's important not to provide a backdoor to
create additional bool instances, so I think bool should not be
subclassable."

Note that range is a function, not a class, hence the error when inheriting from it.

JM


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