class variables

[Andre Meyer]

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'
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