confusion with decorators

Jason Swails jason.swails at gmail.com
Thu Jan 31 01:34:03 CET 2013


Hello,

I was having some trouble understanding decorators and inheritance and all
that.  This is what I was trying to do:

# untested
class A(object):
   def _protector_decorator(fcn):
      def newfcn(self, *args, **kwargs):
         return fcn(self, *args, **kwargs)
      return newfcn

   @_protector_decorator
   def my_method(self, *args, **kwargs):
      """ do something here """

class B(A):
   def _protector_decorator(fcn):
      def newfcn(self, *args, **kwargs):
         raise MyException('I do not want B to be able to access the
protected functions')
      return newfcn

The goal of all that was to be able to change the behavior of my_method
inside class B simply by redefining the decorator. Basically, what I want
is B.my_method() to be decorated by B._protector_decorator, but in the code
I'm running it's decorated by A._protector_decorator.

I presume this is because once the decorator is applied to my_method in
class A, A.my_method is immediately bound to the new, 'decorated' function,
which is subsequently inherited (and not decorated, obviously), by B.

Am I correct here?  My workaround was to simply copy the method from class
A to class B, after which B._protector_decorator decorated the methods in
B.  While this doesn't make the use of decorators completely pointless (the
decorators actually do something in each class, it's just different), it
does add a bunch of code duplication which I was at one point hopeful to
avoid.

I'm still stumbling around with decorators a little, but this exercise has
made them a lot clearer to me.

Thanks!
Jason
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