[Tutor] slots and inheritance - a bit more

[Arthur Siegel]

> I'm not sure what you mean.  The goal of slots is to save memory, and the
> more-efficient memory layout at the C level is inherited (indeed, that's
why
> you can't inherit from more than one base class with __slots__).

>From What's New in Python 2.2:

>>Finally, it's possible to constrain the list of attributes that can be
referenced on an >>object using the new __slots__ class attribute. Python
objects are usually very >>dynamic; at any time it's possible to define a
new attribute on an instance by just >>doing obj.new_attr=1. This is
flexible and convenient, but this flexibility can also >>lead to bugs, as
when you meant to write obj.template = 'a' but made a typo and >>wrote
obj.templtae by accident.

No reading of the What's New could find any indication that what you
describe as a side effect is other than main purpose of slots.  Though I
happen
to believe you.

And it is true that Guido's http://www.python.org/2.2/descrintro.html only
talks of slots in connection with memory issues.

So I'll settle for a refund only on the What's New portion of my investment.

> [Arthur Siegel]
> > But the *functionality* of slots is not inherited
>
> > - unless I kill B's __dict__.  Correct? If so, is that doable?
>
> In a single-inheritance hierarchy, you can specify __slots__ at all, no,
or
> any subset of levels.  Instances of a level that defines __slots__  have
no
> dicts; instances of a level that doesn't have __slots__ do have dicts.
For
> example,
>
> class A(object):
>     __slots__ = 'a'
>
> class B(A):
>     __slots__ = []
> Then an instance of B has no dict (because it *also* specifies __slots__,
> albeit an empty set of slots), and can only use the inherited 'a' as an
> attribute name.

I would have thought that B's empty __slots__ would override A's __slots__
and leave B with no attribute slots.

The fact is - as you tutor - it is otherwise.

B's __slot__ declaration, in my terminology, "appends" to __slots__
of A - is what I'm learning.

>>> class A(object):
 __slots__=("a")

>>> class B(A):
 __slots__=("b")

>>> b=B()
>>> b.a=4
>>> print b.a
4

Now, I might get to where I was hoping to get with slots. No objection here
to taking advantage of a side-effect.  Basically had concluded I need to
build
an optlist for my **kw args.  __slots__ might end up working as that -  with
some
side benefits.

> > But if you mean the multi-heritance restriction - yes.  Happens to be a
> > deal killer in my case.
>
> Damn!  We'll send you a refund check at close-of-business Monday <wink>.

Knowing what I know now, slots might yet do it for me.  Hold the check
(escrowed under the normal arrangements).


Art