On 2021-12-12 16:22, Steven D'Aprano wrote:
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).
You're still missing what people have mentioned a few times. It is
true that this annotates an attribute:
class Foo:
x: int
The question is what does this annotate:
Annotated[int, "some text here"]
In other words what does the use of Annotated in itself annotate. As
Paul pointed out in his earlier message, you can create this Annotated
thing without attaching it to any attribute. It seems that Annotated
itself is annotated the type. In fact, there can be no debate about
what Annotated annotates, since the documentation even says it
explicitly (https://docs.python.org/3/library/typing.html#typing.Annotated):
> Specifically, a type T can be annotated with metadata x via the
typehint Annotated[T, x].
So yes, Annotated IS annotating the type. The text specified in
`Annotated[T, text]` is creating a sort of augmented type, which is like
"a type that is type T but additionally means such-and-such". It is
true that that can type can itself later be used to annotate an
attribute (or variable), but at that point it will be marking the
variable as being of a type that incorporates the annotating text, not
annotating the variable itself with that text.
As such, I don't think Annotated is really a good choice for this. We
really want the text of the annotation to be associated with the
attribute itself, not with the attribute's type. Moreover, using
Annotated requires the user to specify a type, but people should be able
to specify annotations for documentation even if they're not using
typing at all and not specifying any types. (They shouldn't have to
resort to an awkward workaround like always throwing in a dummy Any type.)
As an aside, seeing what the docs say about Annotated makes me think
that "Annotated" is a very bad name for this thing. It confuses the
idea of a type annotation (i.e., attached to a variable) with this
type-incorporating-a-label, where neither the label nor the type is
actually an annotation in the type-annotation sense (because they have
not been attached to a variable to annotate it). It seems it would have
been better to called Annotated "Tagged" or "Labeled" or some such thing
to make it clear that when you using it you are defining a new
special-purpose type for use in later annotations./
--
Brendan Barnwell
"Do not follow where the path may lead. Go, instead, where there is no
path, and leave a trail."
--author unknown
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