Decorating class methods - how to pass the instance to the decorator?

This is Python 2.5, and it's GAE too, not that it matters.

I have the following code. I'm decorating the foo() method in bar, using the dec_check class as a decorator.

class dec_check(object):

  def __init__(self, f):
    self.func = f

  def __call__(self):
    print 'In dec_check.__init__()'
    self.func()

class bar(object):

  @dec_check
  def foo(self):
    print 'In bar.foo()'

b = bar()
b.foo()

When executing this I was hoping to see:

In dec_check.__init__()
In bar.foo()

But I'm getting TypeError: foo() takes exactly 1 argument (0 given) as .foo(), being an object method, takes self as an argument. I'm guessing problem is that the instance of bar doesn't actually exist when I'm executing the decorator code.

So how do I pass an instance of bar to the decorator class?

You need to make the decorator into a descriptor -- either by ensuring its (meta)class has a __get__ method, or, way simpler, by using a decorator function instead of a decorator class (since functions are already descriptors). E.g.:

def dec_check(f):
  def deco(self):
    print 'In deco'
    f(self)
  return deco

class bar(object):
  @dec_check
  def foo(self):
    print 'in bar.foo'

b = bar()
b.foo()

this prints

In deco
in bar.foo

as desired.

Alex's answer suffices when a function is sufficient. However, when you need a class you can make it work by adding the following method to the decorator class.

def __get__(self, obj, objtype):
    """Support instance methods."""
    import functools
    return functools.partial(self.__call__, obj)

To understand this you need to understand the descriptor protocol. The descriptor protocol is the mechanism for binding a thing to an instance. It consists of __get__, __set__ and __delete__, which are called when the thing is got, set or deleted from the instances dictionary.

In this case when the thing is got from the instance we are binding the first argument of its __call__ method to the instance, using partial. This is done automatically for member functions when the class is constructed, but for a synthetic member function like this we need to do it explicitly.

If you want to write the decorator as a class you can do:

from functools import update_wrapper, partial

class MyDecorator(object):
    def __init__(self, func):
        update_wrapper(self, func)
        self.func = func

    def __get__(self, obj, objtype):
        """Support instance methods."""
        return partial(self.__call__, obj)

    def __call__(self, obj, *args, **kwargs):
        print('Logic here')
        return self.func(obj, *args, **kwargs)

my_decorator = MyDecorator

class MyClass(object):
     @my_decorator
     def my_method(self):
         pass