Christian Kaula
Stuff made simple.

Python: Decorate a Method That Gets Passed the Class Instance

The other day I needed to decorate a method with a decorator class that knows about the owner of the method meaning the instance. This works as expected with function decorators:

def some_decorator(func):
  def decorator(self, *args, **kwargs):
    print 'instance %s of class %s is now decorated whee!' % (
      self, self.__class__
    )
    return func(*args, **kwargs)

  return decorator


class SomeClass(object):
  @some_decorator
  def dostuff(self, foo, bar):
    print 'do %s, %s' % (foo, bar)

If you need/want to use a class based decorator you have to do this:

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

def __call__(self, *args, **kwargs):
  print 'instance %s of class %s this is now decorated whee!' % (
    self.obj, self.cls
  )
  return self.func.__call__(*args, **kwargs)

def __get__(self, instance, owner):
  self.cls = owner
  self.obj = instance

  return self.__call__


class SomeClass(object):
@SomeDecorator
def dostuff(self, foo, bar):
  print 'do %s, %s' % (foo, bar)

Thanks to Ozgur who’s post about the topic pointed me in the right direction.