Deprecated since 3.9. collections.abc.Callable now supports subscripting ([]). See PEP 585 and Generic Alias Type.
Deprecated alias to collections.abc.Callable.
Callable[[int], str] signifies a function that takes a single
parameter of type int and returns a str.
The subscription syntax must always be used with exactly two
values: the argument list and the return type.
The argument list must be a list of types, a ParamSpec,
Concatenate or ellipsis. The return type must be a single type.
There is no syntax to indicate optional or keyword arguments;
such function types are rarely used as callback types.