[Python-Dev] Meta-reflections (original) (raw)

Samuele Pedroni pedroni@inf.ethz.ch
Tue, 19 Feb 2002 18:29:55 +0100

Hi.

From: Kevin Jacobs <jacobs@penguin.theopalgroup.com>

On 18 Feb 2002, Martin v. Loewis wrote: > Kevin Jacobs <jacobs@penguin.theopalgroup.com> writes: > > 2) Should attribute access follow the same resolution order rules as > > methods? > > Yes, I think so.

Ouch! This implies a great deal more than you may be thinking of. For example, do you really want to be able to do this: class Foo(object): slots = ('a',) class Bar(Foo): slots = ('a',) bar = Bar() bar.a = 1 super(Bar, bar).a = 2 print bar.a > 1 This violates the traditional Python idiom of having a flat namespace for attributes, even in the presence of inheritance. This has very profound implications to Python semantics and performance.

Probably I have not followed the discussion close enough. The variant with super does not work but

bar=Bar() bar.a=1 Foo.a.set(bar,2) bar.a 1 Foo.a.get(bar) 2

works. Slots are already not flat. They have basically a similar behavior to fields in JVM object model (and I presume in .NET).

Given that implementing slots with fields is one of the possibility for Jython (and some possible Python over .NET), with indeed some practical advantages [Btw for the moment I don't see obstacles to such an approach but I have not considered all the details], it is probably reasonable to keep things as they are.

Consider also:

class Goo(object): ... slots = ('a',) ... class Bar(Goo,Foo): pass ... Traceback (most recent call last): File "", line 1, in ? TypeError: multiple bases have instance lay-out conflict

that helps and reinforces that model.

Samuele.