Nesting descriptors/decorators in python

Question

I'm having a hard time understanding what happens when I try to nest descriptors/decorators. I'm using python 2.7. For example, let's take the following simplified versions of `property` and `classmethod`: ``` class MyProperty(object): def __init__(self, fget): self.fget = fget def __get__(self, obj, objtype=None): print 'IN MyProperty.__get__' return self.fget(obj) class MyClassMethod(object): def __init__(self, f): self.f = f def __get__(self, obj, objtype=None): print 'IN MyClassMethod.__get__' def f(*args, **kwargs): return self.f(objtype, *args, **kwargs) return f ``` Trying to nest them: ``` class A(object): # doesn't work: @MyProperty @MyClassMethod def klsproperty(cls): return 555 # works: @MyProperty def prop(self): return 111 # works: @MyClassMethod def klsmethod(cls, x): return x**2 % print A.klsproperty IN MyProperty.__get__ ... TypeError: 'MyClassMethod' object is not callable ``` The `__get__` method of the inner descriptor `MyClassMethod` is not getting called. Failing…

Accepted Answer

You can make your code work if you make `MyProperty` apply the descriptor protocol to its wrapped object:

```
class MyProperty(object):
    def __init__(self, fget):
        self.fget = fget
    def __get__(self, obj, objtype=None):
        print('IN MyProperty.__get__')
        try:
            return self.fget.__get__(obj, objtype)()
        except AttributeError: # self.fget has no __get__ method
            return self.fget(obj)
```

Now your example code works:

```
class A(object):
    @MyProperty
    @MyClassMethod
    def klsproperty(cls):
        return 555

print(A.klsproperty)
```

The output is:

```
IN MyProperty.__get__
IN MyClassMethod.__get__
555
```

Nesting descriptors/decorators in python

Solution

Problem