On Wed, 6 Jul 2016 01:02 am, Ian Kelly wrote: > On Mon, Jul 4, 2016 at 2:34 AM, Lawrence D’Oliveiro > <lawrenced...@gmail.com> wrote: >> On Monday, July 4, 2016 at 7:58:07 PM UTC+12, dieter wrote: >>> --> "type(obj)" or "obj.__class__" (there are small differences) >>> give you the type/class of "obj". >> >> When would it not be the same? > > I think the only remaining difference in Python 3 is that > obj.__class__ is assignable and type(obj) is not. For most uses, > type(obj) would be preferred though, in the same way that len(obj) is > preferable to obj.__len__().
Assigning to __class__ will change the result of type(obj). py> class A(object): ... pass ... py> class B(object): ... pass ... py> a = A() py> type(a) <class '__main__.A'> py> a.__class__ = B py> type(a) <class '__main__.B'> This is a deliberate design feature! Provided your classes are designed to be compatible, you can safely change an instance of class A into an instance of class B, and then have a.method() correctly call B.method. (Objective C makes *extensive* use of this technique in Cocoa's standard library functions, but entirely behind the scenes. Apple's semi-official stance on this is that they are allowed to do it because the know what they're doing, but anyone else who tries it is a muppet.) I don't know what this technique is called, but I'm going to call it adoption (as opposed to adaption) -- your A instance gets adopted by class B, after which it behaves like any other B instance. Again, I stress that A and B must be compatible (and written in Python, you can't do it with classes written in C), otherwise their methods won't find the attributes they expect. -- Steven “Cheer up,” they said, “things could be worse.” So I cheered up, and sure enough, things got worse. -- https://mail.python.org/mailman/listinfo/python-list
