class variables

[faulkner]

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.