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Date: Thu, 05 Jan 2012 12:09:41 +0100
From: Jean-Michel Pichavant <jeanmichel@sequans.com>
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To: Peter <peter.milliken@gmail.com>
Subject: Re: can a subclass method determine if called by superclass?
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Xref: x330-a1.tempe.blueboxinc.net comp.lang.python:18546

Peter wrote:
> Situation: I am subclassing a class which has methods that call other
> class methods (and without reading the code of the superclass I am
> discovering these by trial and error as I build the subclass - this is
> probably why I may have approached the problem from the wrong
> viewpoint :-)).
>
> Problem: when overriding one of these "indirectly called" superclass
> methods I would like to take differing actions (in the subclass
> instance) depending on whether it is the superclass or the subclass
> instance performing the call.
>
> Question: Is there any way to determine in a method whether it is
> being called by the superclass or by a method of the subclass
> instance?
>
> Now I suspect that what I am doing is actually very muddy thinking :-)
> and I don't want to attempt to explain why I am approaching the design
> this way as an explanation would require too much work - I will
> consider an alternative inheritance approach while waiting an answer,
> but the answer to the question interested me (even if I do a redesign
> and come up with a more "elegant" approach to the problem).
>
> Thanks
> Peter
>   
As you suspected, this is probably the wrong approach.

However since you asked for a solution anyway :o)

class Parent(object):
  def foo(self):
    # implementation by subclasses is still REQUIRED
    if self.__class__ is Parent:
       raise NotImplementedError()
    # common code for all foo methods
    print "calling foo"

class Child(Parent):
  def foo(self):
    # You can still call the virtual method which contains some code 
    Parent.foo(self)
    # here the custom code


p = Parent()
c = Child()


c.foo()
p.foo()

Note that this is not the best approach, still acceptable because there 
is no code specific to a subclass in the base class.

JM