[Tutor] classproperty for Python 2.7 (read-only enough)

[Steven D'Aprano]

On Wed, Apr 19, 2017 at 09:28:26AM +0200, Thomas Güttler wrote:

[code for a classproperty]

> Nice, if it is that simple.
> 
> Is there a reason why this is not in the standard library?

I haven't had a chance to test Peter's classproperty code yet, but I 
don't expect it to be that simple. People have asked for it before, and 
even Guido himself (the inventor of Python) has agreed that if it 
existed he'd use it, but the proposals have (so far) always stumbled on 
two factors:

- there are not a lot of uses for classproperty that ordinary property 
  isn't "good enough" for;

- its hard to get classproperty to work right.


The *easy* part is to do something like this:


class Spam(object):
    @classproperty
    def x(cls):
        return "some value"


Now you can say:

Spam.x

and it will return "some value". BUT if you say:

Spam.x = "hello world"

the class property doesn't run, and Python just overrides x with the new 
value, and you lose the class property and get just a regular attribute. 
That's bad.

There is a solution to that: use a custom metaclass. And yes, that is 
not just advanced, but Black Magic and Voodoo advanced. But if you do 
it, you can solve the above problem.

However, the metaclass solution creates a *new* problem. We'd like this 
to work too:

obj = Spam()
obj.x  # should call the classproperty and return "some value"


but with the metaclass solution, that doesn't work.

As I said, I haven't had a chance to try Peter's code, so it's possible 
that he's solved all these problems. I'm judging by previous 
discussions.

On the bug tracker there's currently a request for classproperty, but 
its languishing:

https://bugs.python.org/issue24941

See Nick Coghlan's comments in particular:

https://mail.python.org/pipermail/python-ideas/2011-January/008959.html


Here's a previous request that was closed for lack of progress and 
interest:

https://bugs.python.org/issue23586


Here's Guido expressing some interest:

https://mail.python.org/pipermail/python-ideas/2011-January/008955.html


-- 
Steve