Decorating class member functions

[7stud]

On May 3, 7:21 pm, Andy Terrel <andy.ter... at gmail.com> wrote:
> Okay does anyone know how to decorate class member functions?
>
> The following code gives me an error:
>
> Traceback (most recent call last):
>   File "decorators2.py", line 33, in <module>
>     s.update()
>   File "decorators2.py", line 13, in __call__
>     retval = self.fn.__call__(*args,**kws)
> TypeError: update() takes exactly 1 argument (0 given)
>
> ------------------
>
> #! /usr/bin/env python
>
> class Bugger (object):
>     def __init__ (self, module, fn):
>         self.module = module
>         self.fn = fn
>
>     def __call__ (self,*args, **kws):
>         ret_val = self.fn(*args,**kws)
>         return ret_val
>
> def instrument (module_name):
>     ret_val = lambda x: Bugger(module_name, x)
>     return ret_val
>
> class Stupid:
>     def __init__(self):
>         self.val = 1
>
>     @instrument("xpd.spam")
>     def update(self):
>         self.val += 1
>
> s = Stupid()
> s.update()

As far as I can tell, the problem is that the decorator executes when
the class is parsed, and at that time there is no self(the instance
object).  The decorator produces a callable Bugger object, but the
callable object has no way to get self when s.update() is called.
Normally when you call a class function, like s.update(), the
__get__() method in the 'update' function object is called (all
function objects have a __get__() method and therefore are
descriptors).  Then __get__() creates a method object out of the
function object(update), and python automatically passes the instance
object to the method object.  However, the Bugger object does not have
a __get__() method, so no method object is created when the Bugger
object is called, and therefore self is not automatically passed to
the Bugger object.

The following solution adds a __get__() method to the Bugger object.
Python automatically passes the instance object to the __get__()
method, and the solution stores the instance in the Bugger object.
Then __call__ is defined  to send the instance object to update().

class Bugger (object):
    def __init__ (self, module, fn):
        self.module = module
        self.fn = fn

    def __call__ (self,*args, **kws):
        ret_val = self.fn(self.obj, *args,**kws)
        return ret_val

    def __get__(descr, inst, instCls=None):
        descr.obj = inst
        return descr

def instrument (module_name):
    ret_val = lambda func: Bugger(module_name, func)
    return ret_val

class Stupid(object):
    def __init__(self):
        self.val = 1

    @instrument("xpd.spam")
    def update(self):
        self.val += 1

s = Stupid()
s.update()
s.update()
s.update()
print s.val

--output:--
4