Cannot extend with more TypeVars generic class with __init_subclass__

[gsakkis]

Bug report

Bug description:

class Parent[T]:
    def __init_subclass__(cls, **kw):
        pass


class Child[S, T](Parent[T]):
    pass


Child[int, float]

Output:

TypeError: Too many arguments for <class '__main__.Child'>; actual 2, expected 1

CPython versions tested on:

3.12

Operating systems tested on:

macOS

[picnixz]

Ah I remember this one. It's because the parent's __init_subclass__ is not called.

See #139905. However, it appears that we didn't catch this case as we don't have the nicer error message.

[picnixz]

I'm going to close this one as wont fix because it's working as expected but we can improve the error message once again (but as part of the other issue).

cc @StanFromIreland

[StanFromIreland]

See #139905. However, it appears that we didn't catch this case as we don't have the nicer error message.

Did you test? I'm away but the report used 3.12.

[picnixz]

I tested with on 3.15 and yes it's still like that. Let me check again (maybe I used an incorrect python executable).

EDIT: I can confirm that the latest main still suffers from this. The problem is in _check_generic_specialization.

[gsakkis]

It's because the parent's init_subclass is not called.

Actually it is called, just add a print in the body to verify.

Additionally, it's not that any generic parent with __init_subclass__ fails; it's only when the child adds more typevars. The following works fine:

class Parent[T]:
    def __init_subclass__(cls, **kw):
        print("parent")


class Child[T](Parent[T]):
    def __init_subclass__(cls, **kw):
        print("child")


class GrandChild(Child[int]):
    pass

# prints:
# parent
# child

[picnixz]

I'm sorry, I meant that you need to call super().__init_subclass__(**kw) in Parent.__init_subclass__ (implicitly, Parent inherits from Generic). We might need to document this though because it's not exactly straightforward (it is explained in the PEP that [T] is synctatic sugar for inheritance from Generic[T] but it's good to recall this somewhere)

[gsakkis]

Ah interesting, TIL. I sort of knew/assumed that [T] is syntax sugar for Generic, what I didn't know was that Generic has an (apparently non empty) __init_subclass__ that should be called with super(). I need to go doublecheck all my __init_subclass__ implementations and add any missing super calls.