Support magic method init_subclass & class_getitem for extension class (PEP 487, 560)

[KmolYuan]

#3518 supports to creating a generic Python class in Cython but extension class are still not available.
Same as the inheritance arguments support.

cdef class G:
    def __class_getitem__(cls, item):
        return cls

>>> G[int]
TypeError: __class_getitem__() takes exactly 1 positional argument (0 given)

cdef class G:
    def __class_getitem__(cls, *args):
        return cls

>>> G[int]
<class 'int'>

The generated code is incorrect:

static PyObject *xxx__class_getitem__(CYTHON_UNUSED PyObject *__pyx_self, PyObject *__pyx_v_cls, CYTHON_UNUSED PyObject *__pyx_v_item);

Solution

Explicitly add @classmethod for the magic methods. (but actually not required)

cdef class G:
    @classmethod
    def __init_subclass__(cls, a=None, **kwargs):
        super().__init_subclass__(**kwargs)
        print(a)

    @classmethod
    def __class_getitem__(cls, items):
        return cls

>>> G[int]
<class 'xxx.G'>
>>> class A(G[int], a=60): pass
60

[da-woods]

__init_subclass__ looks like a bit of a nightmare for cdef class - it'd probably work when you base a regular class from it, but basing another cdef class off I don't see how it could work. I'd be inclined to have Cython raise an error if you create __init_subclass__ for a cdef class rather than try to work around that inconsistency.

__class_getitem__ should probably work. PR welcome I suspect.