On Sun, Dec 12, 2021 at 05:34:33PM -0800, Paul Bryan wrote:
> 1. While I agree that assigning the `Annotated` value to `SomeType`
> makes `SomeType` a type alias, what do you call the specific instance
> of `Annotated[...]` itself?
Its a type. The specific type is a private implementation detail:
>>> type(Annotated[int, 'something'])
<class 'typing._AnnotatedAlias'>
I just call it a type alias. If you want to be precise, it is an alias
to int, augmented with some extra metadata.
> To date, I've been referring to it as a
> type, but that's also muddying the waters here.
No no, it is absolutely a type! _AnnotatedAlias is an instance of type
itself:
>>> type(typing._AnnotatedAlias)
<class 'type'>
>>> isinstance(typing._AnnotatedAlias, type)
True
although Python plays some undocumented(?) shenanigans to make
issubclass fail:
>>> issubclass(Annotated[int, 'something'], type)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: issubclass() arg 1 must be a class
> 2. I've been flyng fast and loose with the term "annotate". No help
> from PEP 593, which convolutes the situation with passages such as:
I will have a lot more to say on that in another post, replying to
Brendan. Later.
> Getting concrete:
>
> Coordinate = Annotated[int, "a graph coordinate", ValueRange(-100, 100)]
> ...
> @dataclass
> class Point:
> x: Annotated[Coordinate, "the horizontal coordinate"]
> y: Annotated[Coordinate, "the vertical coordinate"]
PEP 593 is absolutely clear on the fact that the semantics of the
metadata are up to the consumer, and that Python does not require it to
be a type or to be treated as a type. It describes the metadata as
"an arbitrary list of Python values"
so we're fine to shove docstrings in there.
--
Steve
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