Final classes in Python 3.x- something Guido isn't telling me?

Question

This question is built on top of many assumptions. If one assumption is wrong, then the whole thing falls over. I'm still relatively new to Python and have just entered the curious/exploratory phase.

It is my understanding that Python does not support the creating of classes that cannot be subclassed (final classes). However, it seems to me that the bool class in Python cannot be subclassed. This makes sense when the intent of the bool class is considered (because bool is only supposed to have two values: true and false), and I'm happy with that. What I want to know is how this class was marked as final.

So my question is: how exactly did Guido manage to prevent subclassing of bool?

>>> class TestClass(bool):
        pass

Traceback (most recent call last):
  File "<pyshell#2>", line 1, in <module>
    class TestClass(bool):
TypeError: type 'bool' is not an acceptable base type

Related question: http://stackoverflow.com/questions/2172189/why-i-cant-extend-bool-in-python

Answer 1

You could do this only via the C API. Clear the Py_TPFLAGS_BASETYPE bit of the tp_flags of the type object.

Like this: http://svn.python.org/projects/python/trunk/Objects/boolobject.c (vs intobject.c where Py_TPFLAGS_BASETYPE is set).

Answer 2

You can simulate the same effect from Python 3.x quite easily:

class Final(type):
    def __new__(cls, name, bases, classdict):
        for b in bases:
            if isinstance(b, Final):
                raise TypeError("type '{0}' is not an acceptable base type".format(b.__name__))
        return type.__new__(cls, name, bases, dict(classdict))

class C(metaclass=Final): pass

class D(C): pass

will give the following output:

Traceback (most recent call last):
  File "C:\Temp\final.py", line 10, in <module>
    class D(C): pass
  File "C:\Temp\final.py", line 5, in __new__
    raise TypeError("type '{0}' is not an acceptable base type".format(b.__name__))
TypeError: type 'C' is not an acceptable base type