I'm not sure which language did you mean there, but in Python those classes can be extended.

Also, the main reasons for special cases are historical; also, they predate PEP8 by a long time.

Sorry. That's something that was true in 2.x but not anymore. There used to be UserString, UserList, UserDict classes for serving as base classes. We still have those, but, at least now, we can extend the fundamental classes directly.

They are a bit more convenient than the built-ins.

We can inherit from `list`, `dict` etc., but objects of those classes are still not as flexible as "normal" objects. For example, we can't add an attribute to a list using `l = []; l.a = 1`.

I believe that makes them the same as any other class defined with `__slots__`:

    >>> class Foo:
    ...   __slots__ = ('a', 'b')
    ...
    >>> f = Foo()
    >>> f.a = 1
    >>> f.b = 2
    >>> f.c = 3
        Traceback (most recent call last):
          File "<stdin>", line 1, in <module>
        AttributeError: 'Foo' object has no attribute 'c'