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2.5 KiB
2.5 KiB
Annotated
Annotated
attaches arbitrary metadata to a given type.
Usages
Annotated[T, ...]
is equivalent to T
: All metadata arguments are simply ignored.
from typing_extensions import Annotated
def _(x: Annotated[int, "foo"]):
reveal_type(x) # revealed: int
def _(x: Annotated[int, lambda: 0 + 1 * 2 // 3, _(4)]):
reveal_type(x) # revealed: int
def _(x: Annotated[int, "arbitrary", "metadata", "elements", "are", "fine"]):
reveal_type(x) # revealed: int
def _(x: Annotated[tuple[str, int], bytes]):
reveal_type(x) # revealed: tuple[str, int]
Parameterization
It is invalid to parameterize Annotated
with less than two arguments.
from typing_extensions import Annotated
# error: [invalid-type-form] "`typing.Annotated` requires at least two arguments when used in a type expression"
def _(x: Annotated):
reveal_type(x) # revealed: Unknown
def _(flag: bool):
if flag:
X = Annotated
else:
X = bool
# error: [invalid-type-form] "`typing.Annotated` requires at least two arguments when used in a type expression"
def f(y: X):
reveal_type(y) # revealed: Unknown | bool
# error: [invalid-type-form] "`typing.Annotated` requires at least two arguments when used in a type expression"
def _(x: Annotated | bool):
reveal_type(x) # revealed: Unknown | bool
# error: [invalid-type-form]
def _(x: Annotated[()]):
reveal_type(x) # revealed: Unknown
# error: [invalid-type-form]
def _(x: Annotated[int]):
# `Annotated[T]` is invalid and will raise an error at runtime,
# but we treat it the same as `T` to provide better diagnostics later on.
# The subscription itself is still reported, regardless.
# Same for the `(int,)` form below.
reveal_type(x) # revealed: int
# error: [invalid-type-form]
def _(x: Annotated[(int,)]):
reveal_type(x) # revealed: int
Inheritance
Correctly parameterized
Inheriting from Annotated[T, ...]
is equivalent to inheriting from T
itself.
from typing_extensions import Annotated
class C(Annotated[int, "foo"]): ...
# TODO: Should be `tuple[Literal[C], Literal[int], Literal[object]]`
reveal_type(C.__mro__) # revealed: tuple[Literal[C], @Todo(Inference of subscript on special form), Literal[object]]
Not parameterized
from typing_extensions import Annotated
# At runtime, this is an error.
# error: [invalid-base]
class C(Annotated): ...
reveal_type(C.__mro__) # revealed: tuple[Literal[C], Unknown, Literal[object]]