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
AndreI 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
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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