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.