Inspecting data descriptor attributes in python

Question

I'm attempting to use a data descriptor to provide some custom get/set functionality to attributes of a class. I'd like to be able to inspect the class at runtime and get a list of data descriptors on that class, and maybe even determine what the type of the descriptor is.

The problem is that when I look at the members I get using inspect.getmembers my data descriptor attributes are resolved (their __get__ method is already called and that result is set as the value of the object).

I'm using the example from: http://docs.python.org/2/howto/descriptor.html

import inspect

class RevealAccess(object):
    """A data descriptor that sets and returns values
       normally and prints a message logging their access.
    """

    def __init__(self, initval=None, name='var'):
        self.val = initval
        self.name = name

    def __get__(self, obj, objtype):
        print 'Retrieving', self.name
        return self.val

    def __set__(self, obj, val):
        print 'Updating', self.name
        self.val = val


class MyClass(object):
    x = RevealAccess(10, 'var "x"')
    y = 5

if __name__ == '__main__':
    for x in inspect.getmembers(MyClass, inspect.isdatadescriptor):
        print x

When I run this, I get:

Retrieving var "x"
('__weakref__', <attribute '__weakref__' of 'MyClass' objects>)

What I expect is more like:

('x', <attribute 'x' of 'MyClass' objects>)
('__weakref__', <attribute '__weakref__' of 'MyClass' objects>)

I know I'm missing something I just can't put my finger on it. Any help appreciated.

warvariuc · Accepted Answer · 2013-06-28 16:10:11Z

3

To get the descriptor itself, you can look into class __dict__:

MyClass.__dict__['x']

But the better way is to modify the getter:

def __get__(self, obj, objtype):
    print 'Retrieving', self.name
    if obj is None:  # accessed as class attribute
        return self  # return the descriptor itself
    else:  # accessed as instance attribute
        return self.val  # return a value

Which gives:

Retrieving var "x"
('__weakref__', <attribute '__weakref__' of 'MyClass' objects>)
('x', <__main__.RevealAccess object at 0x7f32ef989890>)

edited Jun 28, 2013 at 16:10

answered Jun 28, 2013 at 16:04

warvariuc

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