David Hirschfield wrote: > Here's an example that's giving me trouble, I know it won't work, but it > illustrates what I want to do: > > class A(object): > _v = [1,2,3] > def _getv(self): > if self.__class__ == A: > return self._v > return super(self.__class__,self).v + self._v > > v = property(_getv) > > class B(A): > _v = [4,5,6] > b = B() > print b.v > > What I want is for b.v to give me back [1,2,3,4,5,6], but this example > gets into a recursive infinite loop, since super(B,self).v is still > B._getv(), not A._getv().
You don't actually need to use properties here if you don't want to -- the Class.v value can be calculated just once, at the time the class statement is executed: >>> class A(object): ... class __metaclass__(type): ... def __init__(cls, name, bases, classdict): ... superclass = cls.mro()[1] ... if superclass is object: ... v = [] ... else: ... v = list(superclass.v) ... if 'v' in classdict: ... v.extend(cls.v) ... cls.v = v ... v = [1, 2, 3] ... >>> class B(A): ... v = [4, 5, 6] ... >>> class C(B): ... pass ... >>> class D(C): ... v = [7, 8, 9] ... >>> A.v [1, 2, 3] >>> B.v [1, 2, 3, 4, 5, 6] >>> C.v [1, 2, 3, 4, 5, 6] >>> D.v [1, 2, 3, 4, 5, 6, 7, 8, 9] >>> B.v is C.v False Note that A.__metaclass__.__init__ is called when the A, B, C and D class statements are executed. This code simply looks at the superclass of class being created, and creates the appropriate v list. Note that this approach means that you only create the lists once, instead of creating them each time your getv() method is called. However, if the list in a superclass changes, the subclass lists will not be automatically updated. HTH, STeVe -- http://mail.python.org/mailman/listinfo/python-list
