On Mon, 15 Apr 2013 19:43:32 +0200, Antoon Pardon wrote: > Op 15-04-13 12:11, Steven D'Aprano schreef: > > >> Python's data model has always been 100% object oriented. Prior to the >> "class/type" unification, it simply had *two distinct* implementations >> of objects: types, which were written in C, and classes, which were >> written in Python. >> >> After unification, the two kinds of object were no longer entirely >> distinct -- you could then subclass types in Python code, using the >> same "class" keyword as you would use for a pure-Python class. >> >> And starting with Python 3, the last vestiges of the distinction have >> disappeared. Now, "class" and "type" are mere synonyms. Both built-in >> types and custom classes use the same mechanism. > > I had gotten my hopes up after reading this but then I tried: > > > $ python3 > Python 3.2.3 (default, Feb 20 2013, 17:02:41) [GCC 4.7.2] on linux2 > Type "help", "copyright", "credits" or "license" for more information. > >>> class vslice (slice): > ... pass > ... > Traceback (most recent call last): > File "<stdin>", line 1, in <module> > TypeError: type 'slice' is not an acceptable base type > > > It seems types and classes are still not mere synonyms.
You are misinterpreting what you are reading. The mere fact that something cannot be subclassed doesn't mean anything. That's just a restriction put on the class by the implementation. It's not even clear that it is a guaranteed language restriction or a mere accident of implementation. With a bit of metaclass trickery, I could equally create a pure-Python class that cannot be easily subclassed. The proof that types and classes are the same in Python 3 is simple: py> class C: ... pass ... py> type(C) is type(int) is type(type) is type True The type of the pure-Python class is type itself. However, even this can be bypassed, using a metaclass! py> class D(metaclass=Meta): ... pass ... py> type(D) is type False py> issubclass(type(D), type) True So when using a metaclass, the type of the class is not necessarily type itself, but it will be a subclass of type. This does not hold in Python 2.x, not for old-style "classic" classes. Classic classes are in a world of their own, distinct from types: # Python 2 py> class C: ... pass ... py> type(C) <type 'classobj'> py> issubclass(type(C), type) False In Python 3, we can expect these two conditions to always hold: * all instances are instances of object; * all classes are instances of type. Notice that this implies that type and object are circularly defined: object, being a class, is an instance of type, but type, being an object, is an instance of object: py> isinstance(type, object) True py> isinstance(object, type) True These two conditions even apply to unsubclassable objects like slice: py> isinstance(slice(1, 5, 2), object) True py> isinstance(slice, type) True -- Steven -- http://mail.python.org/mailman/listinfo/python-list
