> On 26 Apr 2022, at 20:52, Larry Hastings <la...@hastings.org> wrote:
>
>
>
> On 4/25/22 23:56, Ronald Oussoren wrote:
>> A problem with this trick is that you don’t know how large a class object
>> can get because a subclass of type might add new slots. This is currently
>> not possible to do in Python code (non-empty ``__slots__`` in a type
>> subclass is rejected at runtime), but you can do this in C code.
> Dang it! __slots__! Always there to ruin your best-laid plans. *shakes
> fist at heavens*
>
> I admit I don't know how __slots__ is currently implemented, so I wasn't
> aware of this. However! The first part of my proto-PEP already proposes
> changing the implementation of __slots__, to allow adding __slots__ after the
> class is created but before it's instantiated. Since this is so
> late-binding, it means the slots wouldn't be allocated at the same time as
> the type, so happily we'd sidestep this problem. On the other hand, this
> raises the concern that we may need to change the C interface for creating
> __slots__, which might break C extensions that use it. (Maybe we can find a
> way to support the old API while permitting the new late-binding behavior,
> though from your description of the problem I'm kind of doubtful.)
>
I used the term slots in a very loose way. In PyObjC I’m basically doing:
typedef struct {
PyHeapTypeObject base;
/* Extra C fields go here */
} PyObjCClassObject;
Those extra C fields don’t get exposed to Python, but could well be by using
getset definitions. This has worked without problems since early in the 2.x
release cycle (at least, that’s when I started doing this in PyObjC), and is
how one subclasses other types as well.
“Real” __slots__ don’t work when subclassing type() because type is a var
object. That’s “just” an implementation limitation, it should be possible to
add slots after the variable length bit (he says while wildly waving his
hands).
Ronald
—
Twitter / micro.blog: @ronaldoussoren
Blog: https://blog.ronaldoussoren.net/
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