subclassable types

[Wolfgang Maier]

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!

 

[Steven D'Aprano]

Dear all,
I am wondering what the rules are that determine whether a built-in type
is subclassable or not.

The type-designer thought it should, or shouldn't, be.

As examples, why can you base your classes on int or set, but not on
bool or range?

In the case of bool, because of the rule that bool is a singleton
(actually, "doubleton") class. Since a bool subclass would have
instances, that would break the rule that there is only one true bool and
one false bool.

As for range, I'm not sure. It may not be a deliberate decision, but a
side-effect of the implementation. I have a vague recollection of there
being a difference between "heap types" and "non-heap types", which has
probably got something to do with the C implementation.

If that's the case, I see that both Jython and IronPython have treated
this as a language restriction rather than an accident. IronPython
especially is interesting:
.... pass
....
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: cannot derive from IronPython.Runtime.XRange because it is
sealed

Also: can you use introspection to find out whether a type is valid as a
base type?

I don't believe so, but welcome correction. I would just try. If it
succeeds, it will succeed, otherwise it will raise TypeError.