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.