On Apr 25, 2022, at 22:32, Larry Hastings <[email protected]> wrote: > The general shape of it is the same. First, we have some sort of forward > declaration of the class. I'm going to spell it like this: > > forward class C > > just for clarity in the discussion. Note that this spelling is also viable: > > class C > > That is, a "class" statement without parentheses or a colon. (This is > analogous to how C++ does forward declarations of classes, and it was > survivable for them.) Another viable spelling: > > C = ForwardClass()
I like this latter one exactly because as you say it doesn’t require any syntax
changes. In the fine tradition of Python, we could certainly add syntactic
sugar later, but I like that this can be implemented without it and we can see
how the idea plays out in practice before committing to new syntax.
> This spelling is nice because it doesn't add new syntax. But maybe it's less
> obvious what is going on from a user's perspective.
>
> Whichever spelling we use here, the key idea is that C is bound to a
> "ForwardClass" object. A "ForwardClass" object is not a class, it's a
> forward declaration of a class. (I suspect ForwardClass is similar to a
> typing.ForwardRef, though I've never worked with those so I couldn't say for
> sure.)
I haven’t looked deeply at the code for ForwardRef, but I just cracked open
typing.py and noticed this:
class ForwardRef(_Final, _root=True):
"""Internal wrapper to hold a forward reference."""
__slots__ = ('__forward_arg__', '__forward_code__',
'__forward_evaluated__', '__forward_value__',
'__forward_is_argument__', '__forward_is_class__',
'__forward_module__’)
So it seems that you’re almost there already!
> So, technically, this means we could spell the "continue class" step like so:
>
> class C(BaseClass, ..., metaclass=MyMetaClass, __forward__=C):
> ...
Again, nice! Look Ma, no syntax changes.
I don’t know whether the __slots__ issue will break this idea but it is really
... wonderful!
-Barry
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