Python: why can't descriptors be instance variables?
- by Continuation
Say I define this descriptor:
class MyDescriptor(object):
def __get__(self, instance, owner):
return self._value
def __set__(self, instance, value):
self._value = value
def __delete__(self, instance):
del(self._value)
And I use it in this:
class MyClass1(object):
value = MyDescriptor()
>>> m1 = MyClass1()
>>> m1.value = 1
>>> m2 = MyClass1()
>>> m2.value = 2
>>> m1.value
2
So value is a class attribute and is shared by all instances.
Now if I define this:
class MyClass2(object)
value = 1
>>> y1 = MyClass2()
>>> y1.value=1
>>> y2 = MyClass2()
>>> y2.value=2
>>> y1.value
1
In this case value is an instance attribute and is not shared by the instances.
Why is it that when value is a descriptor it can only be a class attribute, but when value is a simple integer it becomes an instance attribute?
