Hacker News Reader

Comment by ansgri | Hacker News Reader

Why? I thought one should prefer immutability. As for typed dicts.. yes, I’m mostly stuck on old python versions, nice reminder.

View on HN

In general, preferring immutability is great. In Python specifically, it can be hard to pull off given that e.g. something as basic as dict does not have a standard immutable equivalent. You inevitably have to rely on conventions - basically saying "this is supposed to be immutable" rather than enforcing it.

You can use TypedDict from `typing_extensions` if your version doesn't have it. You can use a lot of the newer stuff from there, too, especially if you enable `__future__.annotations`.

How old is your Python, though? TypedDict is from 3.8. That was 5 years ago.

You can use:

    > @dataclass(frozen=True)

to create an immutable data class.

While that works (and I use it extensively), it's a bit hacky. You have to use `object.__setattr__` to set attributes in `__init__` or `__post_init__`, which looks so wrong.

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?

Reply on news.ycombinator.com