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Date: Wed, 30 Jan 2013 19:34:03 -0500
Subject: confusion with decorators
From: Jason Swails <jason.swails@gmail.com>
To: python list <python-list@python.org>
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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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<font color=3D"#000000">Hello,</font><div><font color=3D"#000000"><br></fon=
t></div><div><font color=3D"#000000">I was having some trouble understandin=
g decorators and inheritance and all that. =A0This is what I was trying to =
do:</font></div>
<div><font color=3D"#000000"><br></font></div><div><font color=3D"#000000">=
# untested</font></div><div><font color=3D"#000000">class A(object):</font>=
</div><div><font color=3D"#000000">=A0 =A0def _protector_decorator(fcn):</f=
ont></div>
<div><font color=3D"#000000">=A0 =A0 =A0 def newfcn(self, *args, **kwargs):=
</font></div><div><font color=3D"#000000">=A0 =A0 =A0 =A0 =A0return fcn(sel=
f, *args, **kwargs)</font></div><div><font color=3D"#000000">=A0 =A0 =A0 re=
turn newfcn</font></div>
<div><font color=3D"#000000"><br></font></div><div><font color=3D"#000000">=
=A0 =A0@_protector_decorator</font></div><div><font color=3D"#000000">=A0 =
=A0def my_method(self, *args, **kwargs):</font></div><div><font color=3D"#0=
00000">=A0 =A0 =A0 &quot;&quot;&quot; do something here &quot;&quot;&quot;<=
/font></div>
<div><font color=3D"#000000"><br></font></div><div><font color=3D"#000000">=
class B(A):</font></div><div><font color=3D"#000000">=A0 =A0def _protector_=
decorator(fcn):</font></div><div><font color=3D"#000000">=A0 =A0 =A0 def ne=
wfcn(self, *args, **kwargs):</font></div>
<div><font color=3D"#000000">=A0 =A0 =A0 =A0 =A0raise MyException(&#39;I do=
 not want B to be able to access the protected functions&#39;)</font></div>=
<div><font color=3D"#000000">=A0 =A0 =A0 return newfcn</font></div><div><fo=
nt color=3D"#000000"><br>
</font></div><div>The goal of all that was to be able to change the behavio=
r 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, b=
ut in the code I&#39;m running it&#39;s decorated by A._protector_decorator=
.</div>
<div><br></div><div>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, &#39=
;decorated&#39; function, which is subsequently inherited (and not decorate=
d, obviously), by B.</div>
<div><br></div><div>Am I correct here? =A0My workaround was to simply copy =
the method from class A to class B, after which B._protector_decorator deco=
rated the methods in B. =A0While this doesn&#39;t make the use of decorator=
s completely pointless (the decorators actually do something in each class,=
 it&#39;s just different), it does add a bunch of code duplication which I =
was at one point hopeful to avoid.</div>
<div><br></div><div>I&#39;m still stumbling around with decorators a little=
, but this exercise has made them a lot clearer to me.</div><div><br></div>=
<div>Thanks!</div><div>Jason</div><div><br></div>

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