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

Guido van Rossum guido at python.org
Wed Oct 19 22:53:04 CEST 2005

On 10/19/05, Fredrik Lundh <fredrik at pythonware.com> wrote:
> letting "class" inject a slightly magic "self" variable into the class namespace ?
>     class C:
>         foo = property(self.getFoo, self.setFoo, None, 'the foo property')
>         def getFoo(self):
>             return self._foo
>         def setFoo(self, foo):
>             self._foo = foo
> (figuring out exactly what "self" should be is left as an exercise etc)

It's magical enough to deserve to be called __self__. But even so:

I've seen proposals like this a few times in other contexts. I may
even have endorsed the idea at one time. The goal is always the same:
forcing delayed evaluation of a getattr operation without using either
a string literal or a lambda. But I find it quite a bit too magical,
for all values of xyzzy, that xyzzy.foo would return a function of one
argument that, when called with an argument x, returns x.foo. Even if
it's easy enough to write the solution (*), that sentence describing
it gives me goosebumps. And the logical consequence, xyzzy.foo(x),
which is an obfuscated way to write x.foo, makes me nervous.

(*) Here's the solution:

class XYZZY(object):
    def __getattr__(self, name):
        return lambda arg: getattr(arg, name)
xyzzy = XYZZY()

--Guido van Rossum (home page: http://www.python.org/~guido/)

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