Why does declaring a descriptor class in the __init__ function break the descriptor functionality?
descriptor, python
Solution
Accordingly to the documentation:
The following methods only apply when an instance of the class containing the method (a so-called descriptor class) appears in the class dictionary of another new-style class, known as the owner class. In the examples below, “the attribute” refers to the attribute whose name is the key of the property in the owner class’ `__dict__`. Descriptors can only be implemented as new-style classes themselves.
So you can't have descriptors on instances.
However, since the descriptor gets a ref to the instance being used to access it, just use that as a key to storing state and you can have different behavior depending on the instance.
Problem
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" ```