confusion with decorators
Dave Angel
davea at davea.name
Wed Jan 30 20:12:39 EST 2013
On 01/30/2013 07:34 PM, Jason Swails wrote:
> 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.
>
>
I'm certainly not the expert on decorators; I've only used them for
simple things. But I think I can clear up one misconception.
The decorator function will execute while *compiling* the class A, and
the one in class B is unreferenced.
The decorator @_protector_decorator is shorthand for something like
mymethod = _protector_decorator(mymethod)
So by the time the compiler ends with class A, the mymethod has its
final value.
(Note, I've not used a decorator that was defined inside a class, so I'm
probably missing the appropriate A. or self. or cls. overrides.)
But the order of definition is still correct. A decorator executes
once, just after a function is completed.
--
DaveA
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