Passing arguments to `__attrs_init_subclass__`

[tartopohm]

Hello, thanks for the great work!

My question/issue is related to __attrs_init_subclass__ that was recently added. PEP 487 allows for passing arguments (not sure whether that's the proper name) at class definition. The use case, for me, would be to have some sort of abstract class attributes that should be defined by concrete classes.

To make it short, reusing the example from the docs, this works

class Base:
   @classmethod
   def __init_subclass__(cls, foo):
       cls.foo = foo
       print(f"Base has been subclassed by attrs {cls} and foo={cls.foo}.")


class Derived(Base, foo=42):
   pass

>>> Base has been subclassed by attrs <class '__main__.Derived'> and foo=42.

while

import attrs


class Base:
   @classmethod
   def __attrs_init_subclass__(cls, foo):
       cls.foo = foo
       print(f"Base has been subclassed by attrs {cls} and foo={cls.foo}.")


@attrs.define
class Derived(Base, foo=12):
   pass

does not, throwing TypeError: Derived.__init_subclass__() takes no keyword arguments.

Would there be any workaround?

[hynek]

Yeah I was considering to add it, but 24.1 was 8 months overdue so this fell off the wagon, since I wasn’t sure what’s involved.

[tartopohm]

Fair enough. I am completely swamped at the moment, so no promises, but I'll try to look into it if I can find the time.

[hynek]

I had a brief look, but it looks like there is no way to access the x after the class is created. So, I don't think this is even possible to implement without some hairy metaclasses magic. Happy to be corrected, tho!

[Enegg]

How about passing the init arguments to the decorator?

import attrs


class Base:
   @classmethod
   def __attrs_init_subclass__(cls, foo):
       cls.foo = foo
       print(f"Base has been subclassed by attrs {cls} and foo={cls.foo}.")


@attrs.define(metaclass_args={"foo": 12})
class Derived(Base):
   pass