On 4/15/2020 1:52 PM, Ricky Teachey wrote:

Therefore, it seems weird for __init__ not to call super.

I was not part of it, but I know this was heavily discussed during the development of dataclasses prior to 3.7, and it was decided not to do this, at least at THAT time (not saying that can't change, but there were reasons). If Eric V Smith is about I'm sure he could provide insight, or links to the relevant conversations.

Sorry, I've been tied up on other things.

In general, it's not possible to know how to call super.__init__() if you don't a priori know the arguments it takes. That's why dataclasses doesn't guess. The only general-purpose way to do it is to use *args and **kwargs. Since a goal of dataclasses is to use good typing information that works with type checkers, that seems to defeat part of the purpose.

One thing you could do in this scenario is to use a classmethod as an "alternate constructor". This basically gives you any post- and pre- init functionality you want, using any regular parameters or *args and **kwargs as fits your case. This example shows how to do it with *args and **kwargs, but you could just as easily do it with known param names:

from dataclasses import dataclass

class BaseClass:
    def __init__(self, x, y):
        print('initializing base class', x, y)
        self.x = x
        self.y = y

@dataclass
class MyClass(BaseClass):
    a: int
    b: int
    c: int

    @classmethod
    def new(cls, a, b, c, *args, **kwargs):
        self = MyClass(a, b, c)
        super(cls, self).__init__(*args, **kwargs)
        return self

c = MyClass.new(1, 2, 3, 10, 20)
print(c)
print(c.x, c.y)

If you got really excited about it, you could probably write a decorator to do some of this boilerplate for you (not that there's a lot of it in this example). But I don't think this would get added to dataclasses itself.

To Brett's point: has anyone looked to see what attrs does here? When last I looked, they didn't call any super __init__, but maybe they've added something to address this.

Eric