I think the cleaner alternative would be to use a static or class method as an alternative constructor and use the init the dataclass decorator provides for you. Eg something like:

    @dataclass(frozen=True)
    class Foo:
        bar: int
        baz: str

        @classmethod
        def new(cls, bar: int) -> "Foo":
            baz = calculate_baz(bar)
            return cls(bar, baz)

    foo = Foo.new(10)

What is wrong with a static factory method?