[Python-Dev] PEP 520: Preserving Class Attribute Definition Order (round 5) (original) (raw)

Guido van Rossum guido at python.org
Tue Jun 28 13:43:18 EDT 2016

On Tue, Jun 28, 2016 at 10:30 AM, Eric Snow <ericsnowcurrently at gmail.com> wrote:

On Sun, Jun 26, 2016 at 5:55 PM, Guido van Rossum <guido at python.org> wrote:

On Fri, Jun 24, 2016 at 4:37 PM, Nick Coghlan <ncoghlan at gmail.com> wrote: > This version looks fine to me.

Same to me, mostly. I've updated the PEP per everyone's comments [1], except I still haven't dropped the read-only definitionorder constraint. I'll do that when I resolve the open questions, on which I'd like some feedback: * What about slots? In addition to including slots in definitionorder, the options I see are to either ignore the names in slots, put them into definitionorder right after slots, or stick them in at the end (since their descriptors are added afterward). I'm leaning toward the first one, leaving the slot names out of definitionorder since the names aren't actually part of the definition (slots itself is). Doing so doesn't lose any information and more closely reflects the class definition body.

Sounds fine. I guess this means you don't have to do anything special, right?

* Allow setting definitionorder in type()?

I don't see any reason to disallow "definitionorder" in the namespace passed in to the 3 argument form of builtins.type(). Then dynamically created types can have a definition order (without needing to set cls.definitionorder afterward).

Right.

* C-API for setting definitionorder?

I'd rather avoid any extra complexity in the PEP due to diving into C-API support for creating types with a definitionorder. However, if it would be convenient enough and not a complex endeavor, I'm willing to accommodate that case in the PEP. At the same time, at the C-API level is it so important to accommodate definitionorder at class-creation time? Can't you directly modify cls.dict in C? Perhaps it would be safer to have a simple C-API function to set definitionorder for you?

What's the use case even? I think if definition_order is writable then C code can just use PyObject_SetAttrString(, "definition_order", ).

* Drop the "read-only attribute" requirement?

I really like that read-only implies "complete", which is a valuable message for definitionorder to convey. I think that there's a lot to be said for communicating about a value in that way.

But it's still unique behavior, and it's not needed to protect CPython internals.

At the same time, most of the time Python doesn't keep you from fiddling with similar "complete" values (e.g. name, slots), so I see that point too. And since the interpreter (nor stdlib) doesn't rely on definitionorder, it isn't much of a footgun (nor would setting definitionorder be much of an attractive nuisance).

I suppose I'm having a hard time letting go of the attractiveness of "read-only == complete". However, given that you've been pretty clear what you think, I'm more at ease about it. :)

Yeah, it's time to drop it. ;-)

Anyway, thoughts on the above would be helpful. I'll try to be responsive so we can wrap this up.

-eric

[1] https://github.com/python/peps/blob/master/pep-0520.txt

-- --Guido van Rossum (python.org/~guido)

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