[Python-Dev] Definining properties - a use case for class decorators?

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):

    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.


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