[Python-Dev] Definining properties - a use case for class decorators? (original) (raw)

Barry Warsaw barry at python.org
Tue Oct 18 05:10:50 CEST 2005

On Mon, 2005-10-17 at 22:24, Guido van Rossum wrote:

> IMO, there's not enough advantage in having the property() call before > the functions than after.

Maybe you didn't see the use case that Greg had in mind? He wants to be able to override the getter and/or setter in a subclass, without changing the docstring or having to repeat the property() call. That requires us to do a late binding lookup based on a string.

True, I missed that use case. But can't you already support override-ability just by refactoring the getter and setter into separate methods? IOW, the getter and setter isn't overridden, but they call other methods that implement the core functionality and that /are/ overridden. Okay, that means a few extra methods per property, but that still doesn't seem too bad.

If you can think of a solution that looks better than mine, you're a genius.

Oh, I know that's not the case, but it's such a tempting challenge, I'll try anyway :).

class A(object): def init(self): self._x = 0

def set_x(self, x):
    self._set_x(x)

def _set_x(self, x):
    print 'A._set_x()'
    self._x = x

def get_x(self):
    return self._get_x()

def _get_x(self):
    print 'A._get_x()'
    return self._x

x = property(get_x, set_x)

class B(A): def _set_x(self, x): print 'B._set_x()' super(B, self)._set_x(x)

def _get_x(self):
    print 'B._get_x()'
    return super(B, self)._get_x()

a = A() b = B() a.x = 7 b.x = 9 print a.x print b.x

Basically A.get_x() and A.set_x() are just wrappers to make the property machinery work the way you want.

-Barry

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