Thanks Carl and Chris. After reading your comments, and thinking some more
about it, I agree you are both correct that it only makes sense to have a
DeferredObject act like a closure in the scope of its creation. That
really should suffice for anything I could sensibly want; it's enough for
Dask, and it's enough for functional programming languages with laziness.
Moreover, the limited laziness that Python currently has also follows that
rule. I also haven't gotten to writing that needed section of the PEP, but
I need to address generators and boolean shortcutting as limited kinds of
laziness. Obviously, my PEP is much broader than those, but generators
also use lexical scope not dynamic scope.
Possibly in my defense, I think Carl's PEP 690 can do the same thing. :-)
>
>
> I’m not sure what you mean. I don’t think there’s any way PEP 690 can
> introduce dynamic scoping like this. Can you give an example?
>
Let me just withdraw that. I think I might be able to construct something
where a module's top-level code does something perverse with the call
stack where it winds up getting evaluated that can act like dynamic
scoping. I might be wrong about that; but even if it's technically true,
it would need very abnormal abuse of the language (as bad as the lovely
zero-argument super() does :-)).
I don't think this actually has much effect on encompassing late-binding of
default arguments, other than needing to say the arguments contribute to
the scope on a left-to-right basis. But actually, despite writing the
section because of recent discussion, PEP-671 has little to do with why I
want generalized deferreds. I've suggested it on a number of occasions
long before PEP-671 existed (but admittedly always as a passing thought,
not as a detailed proposal... which my draft still is not either).
On one small point Chris mentions, I realized before he commented that
there was no need to actually rebind the deferred argument to force
evaluation of its default. Simply mentioning it would force the
evaluation. I found something similar in PEP-690 that reminded me that
doing this is plenty:
def func(items=[], n=later len(items)):
n # Evaluate the Deferred
items.append("Hello")
print(n)
Of course, that's an extra first line of the function for the motivating
case of emulating the sentinel pattern. But it also does let us *decide*
when each deferred gets "fixed in place". I.e. maybe a, b, and c are all
`later` arguments. We could make the first line `a, b` but only reference
`c` at some later point where it was appropriate to the logic we wanted.
--
Keeping medicines from the bloodstreams of the sick; food
from the bellies of the hungry; books from the hands of the
uneducated; technology from the underdeveloped; and putting
advocates of freedom in prisons. Intellectual property is
to the 21st century what the slave trade was to the 16th.
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