Ron Provost wrote:
> Within an application I'm working on. The app is written in multiple
> layers such that lower layers provided services to higher layers.
> Ideally in such an architecture, the high-level objects know about
> lower-level ones, but lower-level objects know nothing about the
> higher-level ones. There's only one problem. When this software was
> originally wirtten, one of the low-level objects was given knowledge of
> a higher-level object. This creates a really ugly dependency that I
> want to eliminate.
>
> My solution (at least what I'm trying to implement) is a classic one.
> When a low-level routine needs info from a higher-level routine, let the
> higher-level routine provide a callback which the lower-level routine
> can call. In this way, the lower-level routine knows nothing about
> higher-level routines.
>
> However, Python is complaining about my implementation. It raises an
> exception: TypeError: unbound method fn_impl() must be called with X
> instance as first argument (got int instance instead)
>
> For simplicity, I've narrowed it down to a bit of sample code. class X
> is my low-level service.
>
> class X( object ):
> fn = None
>
> @staticmethod
> def callX( n ):
> return X.fn( n )
>
>
> Now, the following global stuff represents my higher-level routines:
>
> def fn_impl( n ): # my callback
> return n + 1
>
> X.fn = fn_impl # register my callback
>
> Now I can do something which forces my callback (fn_impl) to get called
>
> print X.callX( 3 )
>
>
> I think I would get '4' printed but instead get the above error. What
> am I doing wrong?
>
Normally the callback would be an instance variable, not a class
variable, so each instance could have its own callback. Why use a class
at all if all instances use the same callback? So I don't really
understand why callX is a static method either.
How about (untested):
class X(object):
def __init__(self, callback):
self.callback = callback
def callX(self, n):
return self.callback(n)
def fn_impl(n):
return n+1
x = X(fn_impl)
print x.callX(3)
If I'm right, you should see 4 if you paste this into an interpreter.
regards
Steve
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