Based on the discussion, I think I'd go with defaultproperty.
Questions:
- Should this be in builtins, alongside property, or in
a library module? (Oleg suggested propertytools.)
- Do we need a short PEP?
Jim
Jim Fulton wrote:
Guido van Rossum wrote:
On 9/28/05, Jim Fulton [EMAIL PROTECTED] 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.)
but
read-only it ain't! Lazy also doesn't really describe what's going
on.
I agree.
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):
[skip]
I do this often enough
I use it since about 2000 often enough under the name CachedAttribute:
http://cvs.sourceforge.net/viewcvs.py/ppa/qps/qUtils.py
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[1]:
class _LazyAttribute(object):
Yup, the Zope 3 sources have something very similar:
http://svn.zope.org/Zope3/trunk/src/zope/cachedescriptors/property.py?view=markup
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.
Jim
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