Can I get a reference to a Python property?

Question

If I have this:

class foo(object):
    @property
    def bar(self):
        return 0

f = foo()

How do I get a reference to f.bar without actually invoking the method, if this is even possible?

Edited to add: What I want to do is write a function that iterates over the members of f and does something with them (what is not important). Properties are tripping me up because merely naming them in getattr() invokes their __get__() method.

Answer 1

get_dict_attr (below) looks up attr in a given object's __dict__, and returns the associated value if its there. If attr is not a key in that __dict__, the object's MRO's __dict__s are searched. If the key is not found, an AttributeError is raised.

def get_dict_attr(obj,attr):
    for obj in [obj]+obj.__class__.mro():
        if attr in obj.__dict__:
            return obj.__dict__[attr]
    raise AttributeError

For example,

class Foo(object):
    x=1
    def bar(self):
        pass
    @property
    def baz(self):
        return 0

foo=Foo()
print(get_dict_attr(foo,'x'))
# 1
print(get_dict_attr(foo,'bar'))
# <unbound method Foo.bar>
print(get_dict_attr(foo,'baz'))
# <property object at 0xb77c0dc4>
print(get_dict_attr(foo,'y'))
# AttributeError

Note that this is very different than the normal rules of attribute lookup. For one thing, data-descriptors in obj.__class__.__dict__ (descriptors with both __get__ and __set__ methods) normally have precedence over values in obj.__dict__. In get_dict_attr, obj.__dict__ has precedence.

get_dict_attr does not try calling __getattr__.

Finally, get_dict_attr will only work with objects obj which are instances of new-style classes.

Nevertheless, I hope it is of some help.

---

class Foo(object):
    @property
    def bar(self):
        return 0

f = Foo()

This references the property bar:

print(Foo.bar)
# <property object at 0xb76d1d9c>

You see bar is a key in Foo.__dict__:

print(Foo.__dict__['bar'])
# <property object at 0xb775dbbc>

All properties are descriptors, which implies it has a __get__ method:

print(Foo.bar.__get__)
# <method-wrapper '__get__' of property object at 0xb76d7d74>

You can call the method by passing the object f, and the class of f as arguments:

print(Foo.bar.__get__(f,Foo))
# 0

I am fond of the following diagram. Vertical lines show the relationship between an object and the object's class.

When you have this situation:

   Foo                                B
   | Foo.__dict__={'bar':b}           | B.__dict__={'__get__':...}
   |                      \           |      
   f                       `--------> b

f.bar causes b.__get__(f,Foo) to be called.

This is explained in detail here.

Answer 2

One thing you should know is that data descriptors (i.e., properties) only work when they are applied to (new-style) classes (see http://docs.python.org/reference/datamodel.html#implementing-descriptors). Copying them to an object will not create the property on that object. You need to copy them to a (new-style) class to take effect.