Jan Eden wrote: > Hi, > > after updating a program to use new-style classes, I am a bit confused. My > setup looks like this (simplified): > > #Show.py > > import Data > > class Base(object): > ... > > class Page(Data.Page, Base): > ... > > class Author(Data.Author, Base): > ... > > #Data.py > > class Base: > ... > > class Page(Base): > ... > > class Author(Base): > ... > > The module Show.py provides methods, Data.py provides data > attributes. I use the property method in Data.Base for a certain > attribute - but why does this work? > > Data.Base has no base classes, so it is not based on a built-in type: > How can it be a new-style class?
Data.Base is not a new-style class, but the classes in Show are. And properties may appear to work in old-style classes. This document http://www.python.org/2.2.3/descrintro.html#property says, Properties do not work for classic classes, but you don't get a clear error when you try this. Your get method will be called, so it appears to work, but upon attribute assignment, a classic class instance will simply set the value in its __dict__ without calling the property's set method, and after that, the property's get method won't be called either. Here is an old-style class that defines a property: >>> class OldStyle: ... def getA(self): ... print "getA" ... return self.__a ... def setA(self, value): ... print "setA" ... self.__a = value ... a = property(getA, setA) ... >>> o=OldStyle() OldStyle.getA() is called the first time I access o.a: >>> o.a getA Traceback (most recent call last): File "<stdin>", line 1, in ? File "<stdin>", line 4, in getA AttributeError: OldStyle instance has no attribute '_OldStyle__a' Setting a value for a wipes out the property, now it behaves like a normal attribute: >>> o.a = 3 >>> o.a 3 A class that inherits from object and OldStyle is a new-style class and the property works as expected: >>> class NewStyle(object, OldStyle): ... pass ... >>> n=NewStyle() >>> n.a getA Traceback (most recent call last): File "<stdin>", line 1, in ? File "<stdin>", line 4, in getA AttributeError: 'NewStyle' object has no attribute '_OldStyle__a' >>> n.a=3 setA >>> n.a getA 3 Kent _______________________________________________ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
