delegation pattern via descriptor

[Bruno Desthuilliers]

kedra marbun a écrit :
> i'm confused which part that doesn't make sense?
> this is my 2nd attempt to py, the 1st was on april this year, it was
> just a month, i'm afraid i haven't got the fundamentals right yet. so
> i'm gonna lay out how i got to this conclusion, CMIIW
> 
> **explanation of feeling (0) on my 1st post**
> to me, descriptor is a particular kind of delegation, it takes the job
> of coding the delegation by having a contract with programmers that
> the tree meta operations (get, set, del) on attr are delegated to the
> obj that is bound to the attr
> are we agree that descriptor is a kind of delegation?
> 
> the mechanism that makes descriptor works is in __getattribute__,
> __setattr__, __delattr__ of 'object' & 'type'
> 
> now, if i want a single descriptor obj to be delegated to multiple
> tasks, i can't do it since __get__ doesn't get info that can be used
> to determine which task to do
> i must have diff descriptor obj for each task
> 
> class Helper:
> 	def __init__(self, name):
> 		self.name = name
> 	def __get__(self, ins, cls):
> 		if self.name == 'task0': ...
> 		elif self.name == 'task1': ...
> 		else: ...


Replacing such "big switch" code with polymorphic dispatch is one of the 
  goals (and feature) of OO. This should be:


class Task0(object):
     def __get__(self, obj, cls):
         # code here


class Task1(object):
     def __get__(self, obj, cls):
         # code here


class A(object):
     task0 = Task0()
     task1 = Task1()


If you have common code to share between TaskO and Task1 then factor it 
out into a base class.


> if __get__ receives the name, then i could do
> 
> class Helper:
> 	def __get__(self, ins, cls, name):
> 		...
> 
> class a:
> 	task0 = task1 = Helper()


Yuck.