[Python-Dev] Re: Of slots and metaclasses... (original) (raw)

Guido van Rossum guido@python.org
Thu, 28 Feb 2002 13:58:48 -0500

[Kevin Jacobs wrote me in private to ask my position on slots. I'm posting my reply here, quoting his full message -- I see no reason to carry this on as a private conversation. Sorry, Kevin, if this wasn't your intention.]

Hi Guido;

Now that you are back from your travels, I'll start bugging you, as gently as possible, for some insight into your intent wrt slots and metaclasses. As you can read from the python-dev archives, I've instigated a fair amount of discussion on the topic, though the conversation is almost meaningless without your input.

Hi Kevin, you got me to finally browse the thread "Meta-reflections". My first response was: "you've got it all wrong." My second response was a bit more nuanced: "that's not how I intended it to be at all!" OK, let me elaborate. :-)

You want to be able to find out which instance attributes are defined by slots, so that (by combining this with the instance's dict) you can obtain the full set of attribute values. But this defeats the purpose of unifying built-in types and user-defined classes.

A new-style class, with or without slots, should be considered no different from a new-style built-in type, except that all of the methods happen to be defined in Python (except maybe for inherited methods).

In order to find all attributes, you should never look at slots. Your should search the dict of the class and its base classes, in MRO order, looking for descriptors, and then add the keys of the dict as a special case. This is how PEP 252 wants it to be.

If the descriptors don't tell you everything you need, too bad -- some types just are like that. For example, if you're deriving from a list or tuple, there's no attribute that leads to the items: you have to use len and getitem to find out about these, and you have to "know" that that's how you get at them (although the presence of getitem should be a clue).

Why do I reject your suggestion of making slots (more) usable for introspection? Because it would create another split between built-in types and user-defined classes: built-in types don't have slots, so any strategy based on slots will only work for user-defined types. And that's exactly what I'm trying to avoid!

You may complain that there are so many things to be found in a class's dict, it's hard to tell which things are descriptors. Actually, it's easy: if it has a get (method) attribute, it's a descriptor; if it also has a set attribute, it's a data attribute, otherwise it's a method. (Note that read-only data attributes have a descriptor that has a set method that always raises TypeError or AttributeError.)

Given this viewpoint, you won't be surprised that I have little desire to implement your other proposals, in particular, I reject all these:

I'll be the first to admit that some details are broken in 2.2.

In particular, the fact that instances of classes with slots appear picklable but lose all their slot values is a bug -- these should either not be picklable unless you add a reduce method, or they should be pickled properly. This is a bug of the same kind as the problem with pickling time.localtime() (SF bug #496873), so I'm glad this problem has now been entered in the SF database (as #520644). I haven't made up my mind on how to fix this -- it would be nice if slots would automatically be pickled, but it's tricky (although I think it's doable -- without ever referencing the slots variable :-).

I'm not so sure that the fact that you can "override" or "hide" slots defined in a base class should be classified as a bug. I see it more as a "don't do that" issue: If you're deriving a class that overrides a base class slot, you haven't done your homework. PyChecker could warn about this though.

I think you're mostly right with your proposal "Update standard library to use new reflection API". Insofar as there are standard support classes that use introspection to provide generic services for classic classes, it would be nice of these could work correctly for new-style classes even if they use slots or are derived from non-trivial built-in types like dict or list. This is a big job, and I'd love some help. Adding the right things to the inspect module (without breaking pydoc :-) would probably be a first priority.

Now let me get to the rest of your letter.

So I've been sitting on my hands and waiting for you to dive in and set us all straight. Actually, that is not entirely true; I picked up a copy of 'Putting Metaclasses to Work' and read it cover to cover.

Wow. That's more than I've ever managed (due to what I hope can still be called a mild case of ADD :-). But I think I studied all the important parts. (I should ask the authors for a percentage -- I think they've made quite some sales because of my frequent quoting of their book. :-)

Many things you've done in Python 2.2 are much clearer now, though new questions have emerged. I would greatly appreciate it if you would answer a few of them at a time. In return, I will synthesize your ideas with my own and compile a document that clearly defines and justifies the new Python object model and metaclass protocol.

Maybe you can formulate it as a set of tentative clarifying patches to PEPs 252, 253, and 254?

To start, there are some fairly broad and overlapping questions to get started:

1) How much of IBM's SOMobject MetaClass Protocol (SOMMCP) do you want to adapt to Python? For now (Python 2.2/2.3/2.4 time frame)? And in the future (Python 3.0/3000)?

Not much more than what I've done so far. A lot of what they describe is awfully C++ specific anyway; a lot of the things they struggle with (such as the redispatch hacks and requestFirstCooperativeMethodCall) can be done so much simpler in a dynamic language like Python that I doubt we should follow their examples literally.

2) In Python 2.2, what intentional deviations have you chosen from the SOMMCP and what differences are incidental or accidental?

Hard to say, unless you specifically list all the things that you consider part of the SOMMCP. Here are some things I know:

3) Do you intend to enforce monotonicity for all methods and slots? (Clearly, this is not desirable for instance dict attributes.)

If I understand the concept of monotonicity, no. Python traditionally allows you to override methods in ways that are incompatible with the contract of the base class method, and I don't intend to forbid this. It would be good if PyChecker checked for accidental mistakes in this area, and maybe there should be a way to declare that you do want this enforced; I don't know how though.

There's also the issue that (again, if I remember the concepts right) there are some semantic requirements that would be really hard to check at compile time for Python.

4) Should descriptors work cooperatively? i.e., allowing a 'super' call within get and set.

I don't think so, but I haven't thought through all the consequences (I'm not sure why you're asking this, and whether it's still a relevant question after my responses above). You can do this for properties though.

Thanks for the dialogue!

--Guido van Rossum (home page: http://www.python.org/~guido/)