Python note: the difference between __getattr__ and __getattribute__

I tried to overload __setattr__ today, so that when a value is assigned to item.type, I can check whether it is in the set of possible choices. However, I made a mistake by defining a __setattribute__ function instead of __setattr__.

There is no special function named __setattribute__. The only functions that intercepts attribute accesses in Python are __getattr__, __getattribute__ and __setattr__. The difference between __getattr__ and __getattribute is that, __getattr__ is called when the attribute is not in the object's dictionary, while __getatttribute__ is called whenever the attribute is accessed. Therefore, __getattribute__ will make the speed slower. __setattr__ is the same as __getattribute__ in the triggering mechanism -- it intercepts the assignment operation no matter the attribute to modify already exists or not.

Another note about __getattr__ is that the overloaded method must raise attribute error itself or the program may run into unexpected output. For example, there is an overloaded __getattr__ method:

class Foo(object):
   def __getattr__(self, attr):
       if attr == "bar":
          return "bar"

foo = Foo()

Now when we try to see foo.barrrr which doesn't exist, we get None value instead of a thrown attribute error. The code should be corrected into:

class Foo(object):
    def __getattr__(self, attr):
       if attr == "bar":
          return "bar"
       else:
          raise AttributeError, attr