> There are several ways to fix it. The simplest would be to create a new > property object in CC's definition: > > class CC(C): > def set_value(self, val): > if val < 0: > self.__val = 1 > else: > self.__val = val > value = property(C.get_value, set_value)
It won't fix because __val is defined as a class variable in C, but as an instance variable in CC. The set_value method creates a new __val. A typical Python pitfall. C.get_value would still return _C__val. > But an easier, more flexible method would be to use a metaclass which > automatically adds properties for specially-named methods. One example > of such a metaclass is autoprop from Guido van Rossum's descrintro > essay; see <URL:http://www.python.org/2.2.3/descrintro.html>. Yes, but i dislike it either. This powerfull metaclass approach obscures the property creation right in the place. And it supports the tendency to create fat frameworks. A few weeks ago i talked to someone who avoided propertys completely because they seemed to Javaesque to him. I first agreed and understood his ranting about this awkward get_bla, set_blubb stuff. Then I rembebered that the property() function is just a descriptor-creator and one can easily wrap around it for simplification. So i defined defprop() def defprop(name, default = None, modify=None, check=None, types=[]): name = "__"+name def get(obj): if not hasattr(obj,name): setattr(obj,name, default) return getattr(obj,name) def set(obj,value): if types: if not True in [isinstance(value,t) for t in types]: raise TypeError, "Can't assign %s to property %s."%(value,name[2:]) if check: if not check(value): raise TypeError, "Can't assign %s to property %s."%(value,name[2:]) if modify: setattr(obj,name,modify(value)) else: setattr(obj,name,value) return property(get,set,None) I left out to define delete. It was for demonstration purposes only. Now define Your own classes: class X(object): a = defprop("a") b = defprop("b",types=(tuple,list)) c = defprop("c",check = lambda x:hasattr(x,"__len__")) d = defprop("d",default=0) class XX(X): def set_d(val): if val<0: return 1 return val d = defprop("d",default=0,modify=set_d) # overwrite d It would be nice also to refuse the redundant naming a = defprop("a") and write a = defprop() instead. But it's actually not possible at the moment ( or only with deep introspection ). Session on: >>> x = X() >>> x.d 0 >>> xx = XX() >>> xx.d 0 >>> xx.d = -1 >>> xx.d 1 Ciao Kay -- http://mail.python.org/mailman/listinfo/python-list
