Python decorator on instance method

Question

Anyone know what is wrong with this code?

def paginated_instance_method(default_page_size=25):
    def wrap(func):
        @functools.wraps(func)
        def inner(self, page=1, page_size=default_page_size, *args, **kwargs):
            objects = func(self=self, *args, **kwargs)
            return _paginate(objects, page, page_size)
        return inner
    return wrap

class Event(object):
    ...
    @paginated_instance_method
    def get_attending_users(self, *args, **kwargs):
        return User.objects.filter(pk__in=self.attending_list)

I get the following error:

    Traceback (most recent call last):
      File "<console>", line 1, in <module>
      File "/Users/zarathustra/Virtual_Envs/hinge/hinge_services/hinge/api/decorators.py", line 108, in wrap
        def inner(self, page=1, page_size=default_page_size, *args, **kwargs):
      File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/functools.py", line 33, in update_wrapper
        setattr(wrapper, attr, getattr(wrapped, attr))
    AttributeError: 'Event' object has no attribute '__name__'

The reason why I thought this would work is because, through trial and error, I got the following decorator working like a charm for classmethods:

def paginated_class_method(default_page_size=25):
    def wrap(func):
        @functools.wraps(func)
        def inner(cls, page=1, page_size=default_page_size, *args, **kwargs):
            objects = func(cls=cls, *args, **kwargs)
            return _paginate(objects, page, page_size)
        return inner
    return wrap

Answer 1

Your decorator has an extra level of indirection which is throwing things off. When you do this:

@paginated_instance_method
def get_attending_users(self, *args, **kwargs):
    return User.objects.filter(pk__in=self.attending_list)

You are doing this:

def get_attending_users(self, *args, **kwargs):
    return User.objects.filter(pk__in=self.attending_list)
get_attending_users = paginated_instance_method(get_attending_users)

That is what decorators do. Note that paginated_instance_method is called with get_attending_users as its argument. That means that in your decorator, the argument default_page_size is set to the function get_attending_users. Your decorator returns the function wrap, so get_attending_users is set to that wrap function.

Then when you then call Event().get_attending_users() it calls wrap(self), where self is your Event instance. wrap is expecting the argument to be a function, and tries to return a new function wrapping that function. But the argument isn't a function, it's an Event object, so functools.wrap fails when trying to wrap it.

I have a hunch that what you're trying to do is this:

@paginated_instance_method()
def get_attending_users(self, *args, **kwargs):
    return User.objects.filter(pk__in=self.attending_list)

That is, you want paginated_instance_method to take an argument. But even if you want to use the default value of that argument, you still have to actually call paginated_instance_method. Otherwise you just pass the method as the argument, which is not what paginated_instance_method is expecting.

The reason it "worked" for a classmethod is that a classmethod takes the class as the first argument, and a class (unlike an instance) does have a __name__ attribute. However, I suspect that if you test it further you'll find it's not really doing what you want it to do, as it's still wrapping the class rather than the method.

Answer 2

paginated_instance_method is not a decorator, it is a function that returns a decorator. So

@paginated_instance_method()
def get_attending_users(self, *args, **kwargs):

(Note the parentheses.)

Answer 3

This is really easy, but tricky at first view. Look at pep 318.

@dec2
@dec1
def func(arg1, arg2, ...):
    pass

This is equivalent to:

def func(arg1, arg2, ...):
    pass
func = dec2(dec1(func))

You have an extra wrapper, which takes a decorator's args to use it in the wrapped functions (closure design pattern). So your decorator will look like this:

@dec(arg=True)
def func(arg1, arg2, ...):
    pass

Equivalent to:

def func(arg1, arg2, ...):
    pass
func = dec(arg=True)(func)