python != java. when you say "self.v = ...", you mask the class attribute with an instance attribute. say "C1.v = ...".
Colin J. Williams wrote: > Andre Meyer wrote: > > Hi all > > > > I am trying to understand the magic of Python's class variables and > > tried the following code (see below). > > > > Just out of curiosity, I tried to define a property that provides access > > to a seemingly instancae variable which is in fact a class variable. All > > seems to work fine (case 4), but when a custom object is assigned, an > > instance variable is created instead of using theproerty (case 5). > > > > What goes wrong here? What is the difference between cases 4 and 5? How > > can case 5 be fixed? > > > > thanks a lot for your help > > Andre > > > > Code Listing > > ========= > > > > print; print "*** Case 1 ***"; print > > > > class C1(object): > > > > v = None > > > > def __init__(self, value): > > print '-', self.v > > self.v = value > > > > def value(self): > > return self.v > > > > a1 = C1(1) > > b1 = C1(2) > > print a1.value() > > > > print; print "*** Case 2 ***"; print > > > > class C2(object): > > > > v = None > > > > def __init__(self, value): > > print '-', self.v > > self.__class__.v = value > > > > def value(self): > > return self.__class__.v > > > > a2 = C2(1) > > b2 = C2(2) > > print a2.value() > > > > print; print "*** Case 3 ***"; print > > > > class C3(object): > > > > v = 5 > > > > def __init__(self, value): > > print '-', self.v > > self.v = self.v + value > > > > def value(self): > > return self.v > > > > a3 = C3(1) > > b3 = C3(2) > > print a3.value() > > print a3.v > > print a3.__class__.v > > > > print; print "*** Case 4 ***"; print > > > > class V4(list): > > def work(self): > > return 'done' > > > > class C4(object): > > > > def __set_v(self, v): self.__class__.__v = v > > def __get_v(self): return self.__class__.__v > > def __del_v(self): del self.__class__.__v > > v = property(__get_v, __set_v, __del_v, 'make class variable') > > v = V4() > > > > def __init__(self, value): > > > > print '-', self.v > > self.v.append(value) > > print '+', self.v > > > > @classmethod > > def value(self): > > print self.v.work() > > return self.v > > > > > > a4 = C4(1) > > b4 = C4(2) > > print a4.value() > > print a4.v > > print a4.__class__.v > > print a4.v.work() > > > > > > print; print "*** Case 5 ***"; print > > > > class V5(object): > > def __init__(self, i): > > self.i = i > > > > def work(self): > > return 'done', self.i > > > > class C5(object): > > > > def __set_v(self, v): self.__class__.__v = v > > def __get_v(self): return self.__class__.__v > > def __del_v(self): del self.__class__.__v > > v = property(__get_v, __set_v, __del_v, 'make class variable') > > v = None > > > > def __init__(self, value): > > > > print '-', self.v > > self.v = V5(value) > > print '+', self.v > > # print self.__class__.__dict__ > > # print self.__dict__ > > > > @classmethod > > def value(self): > > print self.v.work() > > return self.v > > > > > > a5 = C5(1) > > b5 = C5(2) > > print a5.value() > > print a5.v > > print a5.__class__.v > > print a5.v.work() > > > > > > Output > > ===== > > > > > > *** Case 1 *** > > > > - None > > - None > > 1 > > > > *** Case 2 *** > > > > - None > > - 1 > > 2 > > > > *** Case 3 *** > > > > - 5 > > - 5 > > 6 > > 6 > > 5 > > > > *** Case 4 *** > > > > - [] > > + [1] > > - [1] > > + [1, 2] > > done > > [1, 2] > > [1, 2] > > [1, 2] > > done > > > > *** Case 5 *** > > > > - None > > + <__main__.V5 object at 0x00AFE0D0> > > - None > > + <__main__.V5 object at 0x00AFE110> > > Traceback (most recent call last): > > File "classvariables.py", line 121, in ? > > print a5.value () > > File "classvariables.py", line 115, in value > > print self.v.work() > > AttributeError: 'NoneType' object has no attribute 'work' > > > André, > > I would have expected a5.v to be equal to b5.v, otherwise what is the > value of a class variable? > > I get: > [Dbg]>>> a5.v == b5.v > False > > I hope that one of the wizards will respond. > > Colin W. -- http://mail.python.org/mailman/listinfo/python-list
