Implement PEP-487: customisation of class creation

[fduxiao]

class ABC:
    def __init_subclass__(cls):
        print(cls)


class SubClass(ABC):
    pass

If you run this with python directly, it gives:

<class '__main__.SubClass'>

But after I compile it with cython, it gives:

TypeError                                 Traceback (most recent call last)
<ipython-input-1-a0954f22e2a4> in <module>
----> 1 import test

test.pyx in init test()

TypeError: __init_subclass__() takes exactly one argument (0 given)

How to solve it?

[scoder]

The relevant bits of PEP-487 need to get implemented. Specifically, __init_subclass__() is an implicit class method and must be treated as such, and class creation should call the __init_subclass__() method in older Python versions (before 3.6).

I assume you were using Py3.6 or later for your tests, right?

[fduxiao]

@scoder Yes, and actually with the latest python 3.7.2. I need this feature because metaclass cannot be passed from an ancestor class, so I choose to use __init_subclass__ instead to control the behavior of a class.

[scoder]

PR welcome, seems relatively easy. If you want to give it a try, see the HackerGuide for hints how to get started, and ask on the cython-dev mailing list whenever you feel like needing help.