On Sun, Dec 12, 2021 at 03:38:23PM -0800, Paul Bryan wrote:
> OK, so it's not the type, except it kind of is.
Except it isn't annotating the type, it is annotating the attribute.
We don't annotate *types*:
int: Sequence[str]
That would be a regular variable or attribute called "int" that just
shadowed the builtin type int. We annotate *variables* (including
function parameters and class attributes).
> In my other example, I used `Annotated` to create what I guess is (and
> what you suggested) a pseudo-type? When it was annotated it had
> nothing to do with any attribute. To reiterate:
>
> SomeType = Annotated[str, "some type"]
That's not an annotation. That's a type alias.
Annotations follow a colon, not an equals sign.
> No attribute here (yet). Only when this is applied as a type hint to an
> attribute would it then apply.
>
> class A:
> attr: SomeType
Right. Now you have an annotation, and sure enough, we would find
{'attr': typing.Annotated[str, "some type"]}
in A.__annotations__. It is still annotating the attribute (to be
precise, the attribute name).
> `Annotated` certainly appears to be intended to provide hints about the
> type. In PEP 593, `MaxLen` is an example of prescribing constraints on
> value. It would apply to an attribute if that attribute was annotated
> with it as a type hint.
And until some variable or attribute was actually annotated with it, it
might as well not exist.
Annotated allows us to create new types, by associating arbitrary
metadata with an existing type. The meaning of that metadata is
completely unspecified. So we can say:
Vec = Annotated[List[Tuple[T, T]], MaxLen(10)]
but that has no real meaning until you annotate a variable or attribute,
in which case the Vec type is annotated onto the attribute.
That use-case for Annotated is independent of the proposed use-case
here. Sure you can use it to create new types, or type-aliases. But
that's not the only thing we can use it for.
The meaning of MaxLen(10) is unspecified. To a human reader, we can
guess that it probably means that the list can have a length of no more
than 10. Any tooling that comes across it is supposed to ignore it if it
doesn't know how to interpret it.
So for all we know, MaxLen(10) is not enforced by any tool, and it is
purely there as documentation to the reader: "Please don't make your
vector longer than ten items".
We're not limited to only using Annotated to create new types. In an
annotation, we can associate arbitrary metadata with the annotated
attribute or variable. The interpretation of that metadata is still up
to the consumer. Is it a type restriction? Is it a docstring? Is it
something else? It's entirely up to the tooling.
By the way, the proposal to just follow attributes with a string has
already been considered and rejected:
https://www.python.org/dev/peps/pep-0224/
--
Steve
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