Questions about the use of descriptors.

[Steven W. Orr]

Question 1:

I have a class A with one attribute and I define __get__ and __set__ for that 
class. Then I create another class B that uses it.

Why does B require that the instance of A be a class variable in B and not 
created as an instance variable in __init__?

E.g.,
# This works fine.
class Truth(object):
     def __init__(self):
         self.is_slave = False

     def __get__(self, obj, objtype):
         return self.is_slave

     def __set__(self, obj, val):
         if not self.is_slave and val:
             self.is_slave = val


class TruthHolder(object):
     IsSlave = Truth()

     def set_truth(self):
         self.IsSlave = True

tt = TruthHolder()
print tt.IsSlave
tt.IsSlave = True
print tt.IsSlave
tt.IsSlave = False
print tt.IsSlave


But if I change TruthHolder to not start as a class variable

class TruthHolder(object):
     def __init__(self):
         self.IsSlave = Truth()

     def set_truth(self):
         self.IsSlave = True

it doesn't seem to use descriptor methods of Truth. It's just using the 
default setter and getter of TruthHolder.

Question2:

Is it the case that people only use descriptors for classes with single 
attributes? Or is it more frequent that descriptors are used with classes that 
have multiple attributes?

I feel like this is powerful juju, but I'm not getting when I should be using 
property and when I should be using descriptors.

General note: I see really simple examples in my searches, but I'd like to see 
a good practical example that has just a bit more meat on it.

-- 
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steveo at syslang.net