[Python-Dev] assymetry in descriptor behavior

[David Abrahams]

Guido van Rossum <guido@python.org> writes:

>> I considered these two options:
>> 
>>   option 1:
>>     introduce a separate metaclass for every wrapped class so that we
>>     have a place to stick a property object.
>> 
>>     option 1a:
>>         only do this if the user supplies a special extra template
>>         parameter to the class_<...> declaration
>> 
>>   option 2:
>>     Implement a special property type which allows us to easily
>>     identify property attributes which correspond to Boost.Python static
>>     data members
>> 
>>     Implement a special __setattr__ in the Boost.Python metaclass which
>>     looks up the attribute on the class to see if it has this special
>>     type; if so, it is called, and otherwise the default __setattr__
>>     behavior takes effect.
>> 
>> I was going to go with option 1.  Are you suggesting option 2?
>
> It would seem to be less work than having a separate metaclass for
> each class that has a static data item.  But it's really up to you.

My reasoning was:

    it favors speed and code size over data size.  The only downside I
    can see is that an extra object (the metaclass object) is created
    for every wrapped class.  It's also simpler to implement.

However, I think it might also cost an extra property object per
wrapped class, unless I can find a way to get the class and its
instance to share the property.  Hmm, feels a bit hack-ish to me now
that you mention it.  Maybe option 2 is better after all.  But then,
do I want to implement tp_getattr or tp_getattro?  I don't recall the
differences.

>> Yes, but type is always non-None, it seems.
>
> Yes, that's what I said.
>
>> Look at descrintro.html again.  It says:
>> 
>>     Example: coding super in Python.
>> 
>>     As an illustration of the power of the new system, here's a fully
>>     functional implementation of the super() built-in class in pure
>>     Python. This may also help clarify the semantics of super() by
>>     spelling out the search in ample detail. The print statement at the
>>     bottom of the following code prints "DCBA".
>> 
>>     class Super(object):
>>         def __init__(self, type, obj=None):
>>             self.__type__ = type
>>             self.__obj__ = obj
>>         def __get__(self, obj, type=None):
>>                                ^^^^^^^^^
>>             if self.__obj__ is None and obj is not None:
>>                 return Super(self.__type__, obj)
>>             else:
>>                 return self
>> 
>> Also, PEP 252 says:
>> 
>>     - __get__(): a function callable with one or two arguments that
>>       retrieves the attribute value from an object.
>> 
>> What's that about?  Does __get__ ever get called with just one
>> argument (excluding self)?  If so, when?
>
> When you call it yourself (rather than when it is called as a result
> of __getattribute__).  

When do you call it yourself?

> I suppose this is a bit redundant.

I guess; I don't know.  I'm trying to understand whether "callable
with one or two arguments" is really part of the requirements for use
as a property method, or just something that got thrown in there.

>> >> 3. Is there documentation for __delete__ anywhere?
>> >
>> > Apparently not, but it's easy to guess what it does if you know
>> > __getattr__, __setattr__ and __delattr__.  It's __delete__ and not
>> > __del__ because __del__ is already taken.  In an early alpha release
>> > it was actually __del__, but that didn't work very well. :-)
>> 
>> It's easy enough to guess what it does by instrumenting it, too.
>> However, it's been out in a released Python for several versions now
>> and I get tired of guessing every time I have to write a new one of
>> these ;-).  I think it's time there were some official docs.
>
> So volunteer some.

I'm anxious to, and I will -- just as soon as I can get a clear
picture of what I should say.  Gee, I guess "someone" should document
__getattribute__ while I'm in there.  Are there any other obvious
omissions in that neighborhood?

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com