Steve D'Aprano wrote:
> On Mon, 21 Nov 2016 11:15 pm, Veek M wrote: > >>>>> class Foo(): >> ... pass >> ... >>>>> class Bar(Foo): >> ... pass >> ... >>>>> b = Bar() >>>>> type(b) >> <type 'instance'> > [...] > >> What is going on here? Shouldn't x = EuroDate(); type(x) give >> 'instance'?? Why is 'b' an 'instance' and 'x' EuroDate? >> Why isn't 'b' Bar? > > > It looks like you are running Python 2, and have stumbled across an > annoyance from the earliest days of Python: the "classic", or > "old-style", class. > > Before Python 2.2, custom classes and built-in types like int, float, > dict and list were different. You have just discovered one of the ways > they were different: instances of custom classes all had the same > type, even if the class was different: > > # Python 2 > > py> class Dog: > ... pass > ... > py> class Cat: > ... pass > ... > py> lassie = Dog() > py> garfield = Cat() > py> type(lassie) is type(garfield) > True > py> type(lassie) > <type 'instance'> > > > This is just the most obvious difference between "classic classes" and > types. Some of the other differences: > > - The method resolution order (MRO) is different: the classic class > MRO is buggy for diamond-shaped multiple inheritance, and special > dunder methods like __eq__ are resolved slightly differently. > > - super, properties, class methods and static methods don't work for > classic classes. > > - The metaclass of classic classes is different: > > py> type(Dog) > <type 'classobj'> > py> type(float) > <type 'type'> > > - Attribute lookup for classic classes is slightly different; in > particular, the special __getattribute__ method doesn't work. > > > In Python 2.2, the built-in types (list, dict, float etc) were unified > with the class mechanism, but for backwards compatibility the > old-style classes had to be left in. So Python had two class > mechanisms: > > - "New-style classes", or types, inherit from object, or some > other built-in type, and support properties, etc. > > - "Old-style classes", don't inherit from object, don't support > properties etc. > > > So in Python 2, when you write: > > class Foo: > > or > > class Foo(): > > > you get an old-style class. But when you inherit from object, you get > a new-style class. Classic classes are an obsolete feature from Python > 2. They are removed in Python 3, and things are much simpler. In > Python 3, it doesn't matter whether you write: > > class Foo: > class Foo(): > class Foo(object): > > the result is the same: a new-style class, or type. > > The best thing to do in Python 2 is to always, without exception, > write > > class Foo(object): > > to define your base classes. That will ensure that property, super, > classmethod, staticmethod, __getattribute__, etc. will all work > correctly, and you will avoid the surprises of classic classes. > > > Thanks guys, got it! -- https://mail.python.org/mailman/listinfo/python-list
