[Python-Dev] Declaring setters with getters

[Guido van Rossum]

On 10/31/07, glyph at divmod.com <glyph at divmod.com> wrote:
> As long as we're all tossing out ideas here, my 2¢.  I vastly prefer
> this:
>
> On 02:43 am, steven.bethard at gmail.com wrote:
> >On 10/31/07, Fred Drake <fdrake at acm.org> wrote:
>
> >>        @property.set
> >>        def attribute(self, value):
> >>            self._ignored = value
>
> to this:
> >      @property.set(attribute)
> >      def attribute(self, value):
> >          self._ignored = value
>
> since I don't see any additional expressive value in the latter, and it
> provides an opportunity to make a mistake.

I was expecting this would be brought up, but that just ain't gonna
happen. If you don't repeat the name, the decorator has to root around
in the surrounding scope, which is fraught with peril. Solutions based
on sys._getframe() have been around for years (e.g. several the
Cookbook recipes) and if I had approved of that technique I would have
adopted one long ago.

However I don't approve of it. It has always been and will always
continue to be my position that these are semantically unkosher,
because it means that you can't wrap them in convenience functions or
invoke them in different contexts, and that means that the semantics
are hard to explain.

If you really want another argument, repeating the property name
actually does have an additional use case: you can have a read-only
property with a corresponding read-write property whose name differs.
E.g.

class C(object):

  @property
  def encoding(self): return ...

  @propset(encoding)
  def encoding_rw(self, value): ...

c = C()
c.encoding = "ascii"  # Fails
c.encoding_rw = "ascii"  # Works

I've seen people do this in the filesystem: a read-only version that
may be cached or replicated, and a separate writable version. Reading
the writable version works too, of course.

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