`cast`

cast() takes two arguments, one type and one value, and returns a value of the given type.

The (inferred) type of the value and the given type do not need to have any correlation.

from typing import Literal, cast, Any

reveal_type(True)  # revealed: Literal[True]
reveal_type(cast(str, True))  # revealed: str
reveal_type(cast("str", True))  # revealed: str

reveal_type(cast(int | str, 1))  # revealed: int | str

reveal_type(cast(val="foo", typ=int))  # revealed: int

# error: [invalid-type-form]
reveal_type(cast(Literal, True))  # revealed: Unknown

# error: [invalid-type-form]
reveal_type(cast(1, True))  # revealed: Unknown

# error: [missing-argument] "No argument provided for required parameter `val` of function `cast`"
cast(str)
# error: [too-many-positional-arguments] "Too many positional arguments to function `cast`: expected 2, got 3"
cast(str, b"ar", "foo")

def function_returning_int() -> int:
    return 10

# error: [redundant-cast] "Value is already of type `int`"
cast(int, function_returning_int())

Complex type expressions (which may be unsupported) do not lead to spurious [redundant-cast] diagnostics.

from typing import Callable

def f(x: Callable[[dict[str, int]], None], y: tuple[dict[str, int]]):
    a = cast(Callable[[list[bytes]], None], x)
    b = cast(tuple[list[bytes]], y)

Casting Unknown to Unknown or Any to Any is not considered redundant; these dynamic types may represent different static types, and erroring on this violates the gradual guarantee.

from ty_extensions import Unknown

def f(x: Any, y: Unknown, z: Any | str | int):
    a = cast(dict[str, Any], x)
    reveal_type(a)  # revealed: dict[str, Any]

    b = cast(Any, y)
    reveal_type(b)  # revealed: Any

    c = cast(str | int | Any, z)

    d = cast(y, y)
    e = cast(x, x)

1.8 KiB Raw Blame History

cast

1.8 KiB

Raw Blame History

`cast`