Pass all args to __attrs_pre_init__()

[brendan-simon-indt]

I am subclassing a class that I have no control over, which is written in standard Python. The base class A has a number of arguments that I want to set. Infact there are 3 classes. C (attrs class) is a sublcass of B (attrs class) which is a subclass of A (non-attrs class).

When I instantiate C, I want to pass arguments to B and A.

I implemented class based factory functions (vai @classmethod), which works well for a single level inheritance, but I'm not sure how to use the classmethod of B from the classmethod of A. I figured I'm stuck with duplicating of the B initialisation within the C classmethod and the instantiating class C by passing all the necessary arguments.

All this involves defining a custom __init__() method (https://www.attrs.org/en/stable/init.html#custom-init), but it seems to me that could be avoided by using __attrs_pre_init__(), but unfortunately it seems no arguments are passed to that (i.e. no *args or **kwargs).

Is there a reason why arguments are not passed to attrs_pre_init__(), and only passed to __init__()?

I think it makes sense to pass all arguments to __attrs_pre_init__() so the user can have the flexibility to do what is required (or nothing at all).

    def __attrs_pre_init__(self, *args, **kwargs):
        ...
        super().__init__(*args, **kwargs)
        ...

I can see why it is less relevant for __attrs_post_init__(), but I'm sure someone can think of a use-case where the arguments could be useful (e.g. for arguments that aren't stored as instanace attributes).

[hynek]

Yeah in hindsight that seems an odd omission. I think the expectation was that you're gonna use a custom init for anything more complicated.

[Tyrubias]

I'm interested in implementing this feature but I'm not sure where to start. I see where the call to __atrs_pre_init__ is being added in src/attr/_make.py. Would I change this call to include the arguments being passed into the __init__ method being generated?

[hynek]

Without looking closer, I suspect you’ll have to inspect the __attrs_pre_init__ method and call with with or without based on its arguments. For all I care: do nothing if it’s only one argument (presumably self), pass them if it has more than one.