I want to define a class-level attribute that will be shared by all
subclasses. That is, I want this class, every subclass of this class, every
instance of this class and every instance of every subclass to be able to
read and write to the _same_ slot/variable. But I can't figure out how to
do it.
My attempts so far have led me to using a descriptor as follow, but it
doesn't get me where I want to be:
class sharedClassVar(object):
def __init__(self,x=None):
print "creating"
self.value = x
def __get__(self,ob,cls):
print "getting"
return self.value
def __set__(self,ob,x):
print "setting"
self.value = x
class A( object ):
# install it like this I guess:
cls2 = sharedClassVar(5)
class B( A ):
pass
print A.cls2 # so far so good
o = A()
o2 = B()
print o.cls2 # ok
print o2.cls2 # ok
o.cls2 = 2
print B.cls2 # perfect
print o2.cls2 # ""
o2.cls2 = 3
print A.cls2 # ""
print o.cls2 # ""
# but I need to be able to update the value through the class
# how do I do it?
A.cls2 = 4 # whoops - we just overwrote the descriptor with an integer
A.cls2.__set__(None,4) # AttributeError: 'int' object has no attribute
'__set__'
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