[Python] Why does declaring a descriptor class in the __init__ function break the descriptor...

In class B below I wanted the __set__ function in class A to be called whenever you assign a value to B().a . Instead, setting a value to B().a overwrites B().a with the value. Class C assigning to C().a works correctly, but I wanted to have a separate instance of A for each user class, i.e. I don't want changing 'a' in one instance of C() to change 'a' in all other instances. I wrote a couple of tests to help illustrate the problem. Can you help me define a class that will pass both test1 and test2?

class A(object):
def __set__(self, instance, value):
print "__set__ called: ", value

class B(object):
def __init__(self):
self.a = A()

class C(object):
a = A()

def test1( class_in ):
o = class_in()
o.a = "test"
if isinstance(o.a, A):
print "pass"
else:
print "fail"

def test2( class_in ):
o1, o2 = class_in(), class_in()
if o1.a is o2.a:
print "fail"
else:
print "pass"

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