In article <[EMAIL PROTECTED]>, "Gre7g Luterman" <[EMAIL PROTECTED]> wrote:
> I suppose I was lulled into complacency by how Python makes so many things > look like classes, but I'm starting to realize that they're not, are they? > > I'm writing a C program which handles Python objects in different ways based > on their type. I do a PyInstance_Check(PyObj) to determine if the PyObj is > an instance, but it is returning 0 on a lot of stuff that I thought would be > an instance. So I did the following simple test on three things that look > like instances: > > Python 2.5.1 (r251:54863, Apr 18 2007, 08:51:08) [MSC v.1310 32 bit (Intel)] > on win32 > Type "help", "copyright", "credits" or "license" for more information. > >>> class a: pass > ... > >>> type(a()) > <type 'instance'> > >>> type(Exception()) > <type 'exceptions.Exception'> > >>> class b(dict): pass > ... > >>> type(b()) > <class '__main__.b'> > > I was relieved that a() returns an instance, but I was surprised that > Exceptions aren't really instances at all. And what's the deal with derving > a class from a standard type like a dictionary? I thought for sure, that > would be an instance, but this shows it is a class?!? > There are actually two kinds of classes in Python (as of 2.2): new-style classes and classic classes. What you are calling an instance is an instance of a classic class. Here is how a new-style class would look: >>> class c(object): pass ... >>> type(c()) <class '__main__.c'> Actually the new-style classes are a lot more consistent with the built-in types, and the type hierarchy makes more sense with them, so I would suggest in general migrating towards new-style classes for all of your own code. More information about new-style classes can be found here: http://docs.python.org/ref/node33.html http://www.python.org/doc/newstyle.html > Can anyone explain the last one and/or give me a simple test I can do in C > to determine whether a Python object is "instance-like"? With new-style classes, the type of an instance of that class is the class itself (type(x) == x.__class__). In your above example, class b is a subclass of dict, which itself is a new-style class, making b a new-style class as well. Thus type(b()) is b. As for a check, it really depends on what you mean by "instance-like". Are dictionaries "instance-like"? What about strings? Integers? Dave -- http://mail.python.org/mailman/listinfo/python-list
