Unpickling child class fails if base class __init_subclass__ expects an argument

[Gollum999]

If you have a base class that defines __init_subclass__ with an argument, unpickling a child class will fail because cloudpickle does not pass along the original kwarg when re-creating the class.

This case seems to work fine in the built-in pickle package.

This usage mirrors examples shown in the docs and the original PEP that introduced the __init_subclass__ method.

Python 3.11.4
cloudpickle 2.2.1
EDIT: This also appears to be broken with cloudpickle 3.0.0.

Example:

#!/usr/bin/env python3
import pickle
import cloudpickle
print(cloudpickle.__version__)


class Base:
    def __init_subclass__(cls, /, arg, **kwargs):
        super().__init_subclass__(**kwargs)
        cls.arg = arg


class Child(Base, arg='hello'):
    pass


print(pickle.loads(pickle.dumps(Base)))
print(pickle.loads(pickle.dumps(Child)))
print(cloudpickle.loads(cloudpickle.dumps(Base)))
print(cloudpickle.loads(cloudpickle.dumps(Child)))

2.2.1
<class '__main__.Base'>
<class '__main__.Child'>
<class '__main__.Base'>
Traceback (most recent call last):
  File "/home/tsanders/test_cloudpickle_bug.py", line 20, in <module>
    print(cloudpickle.loads(cloudpickle.dumps(Child)))
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/conda/envs/production-3.11/lib/python3.11/site-packages/cloudpickle/cloudpickle.py", line 827, in _make_skeleton_class
    skeleton_class = types.new_class(
                     ^^^^^^^^^^^^^^^^
  File "/opt/conda/envs/production-3.11/lib/python3.11/types.py", line 75, in new_class
    return meta(name, resolved_bases, ns, **kwds)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: Base.__init_subclass__() missing 1 required positional argument: 'arg'

[kentropy]

I also encountered this issue in Ray that uses cloudpickle to serialise/deserialise ray-project/ray#42823 (comment)

[ogrisel]

Reading https://docs.python.org/3/reference/datamodel.html#object.__init_subclass__ it sounds that this is indeed a bug in cloudpickle but I am not familiar enough with Python internals to know how to fix it right away.

Feel free to investigate yourself and submit a PR.

EDIT: maybe cloudpickle.cloudpickle._class_getnewargs should be made aware of what is returned by obj.__getnewargs_ex__ if this method is defined on the class instance to pickle. However I don't see how that could help pickling a class definition passed to cloudpickle.dumps without passing an instance. It seems that __getnewargs_ex__ is not meant to be a classmethod.