Callable annotation with arguments

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Accepted Answer

Callable[[int, str], bool] means 'a callable that takes an int and a str and returns a bool'. The first type list is the parameter types; the second argument after the comma is the return type. Here, check(2, 'hi') returns len('hi') > 2, which is 2 > 2 = False. Then check(2, 'hello') returns len('hello') > 2, which is 5 > 2 = True. The annotation itself does nothing at runtime - apply() just calls fn(x, s) and returns whatever comes back. For callables that accept any arguments, use Callable[..., bool] with an ellipsis.