[Python-Dev] RFC: readproperty
jim at zope.com
Thu Sep 29 10:20:31 CEST 2005
Guido van Rossum wrote:
> On 9/28/05, Jim Fulton <jim at zope.com> wrote:
> I think we need to be real careful with chosing a name -- in Jim's
> example, *anyone* could assign to Spam().eggs to override the value.
> The name "readproperty" is too close to "readonlyproperty",
In fact, property creates read-only properties for new-style classes.
(I hadn't realized, until reading this thread, that for classic
classes, you could still set the attribute.)
> read-only it ain't! "Lazy" also doesn't really describe what's going
> I believe some folks use a concept of "memo functions" which resemble
> this proposal except the notation is different: IIRC a memo function
> is always invoked as a function, but stores its result in a private
> instance variable, which it returns upon subsequent calls. This is a
> common pattern. Jim's proposal differs because the access looks like
> an attribute, not a method call. Still, perhaps memoproperty would be
> a possible name.
> Another way to look at the naming problem is to recognize that the
> provided function really computes a default value if the attribute
> isn't already set. So perhaps defaultproperty?
Works for me.
Oleg Broytmann wrote:
> On Wed, Sep 28, 2005 at 10:16:12AM -0400, Jim Fulton wrote:
>> class readproperty(object):
>>I do this often enough
> I use it since about 2000 often enough under the name CachedAttribute:
Steven Bethard wrote:
> Jim Fulton wrote:
> I've also needed behavior like this a few times, but I use a variant
> of Scott David Daniel's recipe:
> class _LazyAttribute(object):
Yup, the Zope 3 sources have something very similar:
I actually think this does too much. All it saves me, compared to what I proposed
is one assignment. I'd rather make that assignment explicit.
Anyway, all I wanted with readproperty was a property that implemented only
__get__, as opposed to property, which implements __get__, __set__, and __delete__.
I'd be happy to call it readproprty or getproperty or defaulproperty or whatever. :)
I'd prefer that it's semantics stay fairly simple though.
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