On 26/04/2022 20:48, Carl Meyer via Python-Dev wrote:
On Tue, Apr 26, 2022 at 1:25 PM Guido van Rossum <gu...@python.org> wrote:
I also would like to hear more about the problem this is trying to solve, when
th real-world examples. (E.g. from pydantic?)
Yes please. I think these threads have jumped far too quickly into
esoteric details of implementation and syntax, without critical
analysis of whether the semantics of the proposal are in fact a good
solution to a real-world problem that someone has.
I've already outlined in a more detailed reply on the first thread why
I don't think forward declarations provide a practically useful
solution to forward reference problems for users of static typing
(every module that imports something that might be a forward reference
would have to import its implementation also, turning every one-line
import of that class into two or three lines) and causes new problems
for every user of Python due to its reliance on import side effects
causing global changes at a distance. See
https://mail.python.org/archives/list/python-dev@python.org/message/NMCS77YFM2V54PUB66AXEFTE4NXFHWPI/
for details.
Under PEP 649, forward references are a small problem confined to the
edge case of early resolution of type annotations. There are simple
and practical appropriately-scoped solutions easily available for that
small problem: providing a way to resolve type annotations at runtime
without raising NameError on not-yet-defined names. Such a facility
(whether default or opt-in) is practically useful for many users of
annotations (including dataclasses and documentation tools), which
have a need to introspect some aspects of annotations without
necessarily needing every part of the annotation to resolve. The
existence of such a facility is a reasonable special case for
annotations specifically, because annotations are fundamentally
special: they provide a description of code, rather than being only a
part of the code. (This special-ness is precisely also why they cause
more forward references in the first place.)
IMO, this forward declaration proposal takes a small problem in a
small corner of the language and turns it into a big problem for the
whole language, without even providing as nice and usable an option
for common use cases as "PEP 649 with option for lax resolution" does.
This seems like a case study in theoretical purity ("resolution of
names in annotations must not be special") running roughshod over
practicality.
Carl
Insofar as I understand the above (knowing almost nothing about typing), +1.
Best wishes
Rob Cliffe
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