Over my head with descriptors

[Christian Kastner]

Sarcastic Zombie wrote:
> Code included below.
> 
> Basically, I've created a series of "question" descriptors, which each
> hold a managed value. This is so I can implement validation, and render
> each field into html automatically for forms.
> 
> My problem is this: every instance of my "wizard" class has unique self
> values, but they share the exact same descriptor values.

That's because descriptors only work as class attributes, not instance
attributes.

> Meaning, if
> 
> t = Test("ar")
> y = Test("ar")
> 
> t is y
> False
> 
> t.age is y.age
> True

As an attribute of the owner class, the descriptor is instantiated once
for the class itself, and not for every instance of the owner class:

    class Foo(object):

        # Class attribute
        desc = Descriptor()

        def __init__(self, value):
            # Instance attributes go here
            self.value = somevalue

> Code below. What am I not understanding?
> -----------------------------------------
...
> class Question(object):
...
> 	def __get__(self, instance, owner):
> 		return self

If you want to do per-instance stuff, use the "instance" argument:

    # Warning: simplified
    class Descriptor(object):
        def __get__(self, instance, owner):
            return instance._desc

        def __set__(self, instance, value):
            instance._desc = value

Chris