Can't use __init_subclass__ in the Model subclass directly (not as a mixin class)

[Tatiana Tereshchenko]

There is a closed Django ticket which references PEP and describes the new __init_subclass__ in Python 3.6, it solves the case when __init_subclass__ is used in a mixin class.

Unfortunately, a direct usage of init_subclass in a subclass of Django Model class results in TypeError:

$ python manage.py shell
Traceback (most recent call last):
  File "manage.py", line 15, in <module>
    execute_from_command_line(sys.argv)
  File "/home/tt/django_reproducer/lib64/python3.6/site-packages/django/core/management/__init__.py", line 381, in execute_from_command_line
    utility.execute()
  File "/home/tt/django_reproducer/lib64/python3.6/site-packages/django/core/management/__init__.py", line 357, in execute
    django.setup()
  File "/home/tt/django_reproducer/lib64/python3.6/site-packages/django/__init__.py", line 24, in setup
    apps.populate(settings.INSTALLED_APPS)
  File "/home/tt/django_reproducer/lib64/python3.6/site-packages/django/apps/registry.py", line 112, in populate
    app_config.import_models()
  File "/home/tt/django_reproducer/lib64/python3.6/site-packages/django/apps/config.py", line 198, in import_models
    self.models_module = import_module(models_module_name)
  File "/usr/lib64/python3.6/importlib/__init__.py", line 126, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 994, in _gcd_import
  File "<frozen importlib._bootstrap>", line 971, in _find_and_load
  File "<frozen importlib._bootstrap>", line 955, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 665, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 678, in exec_module
  File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
  File "/home/tt/mysite/polls/models.py", line 25, in <module>
    class MyPluginModel(MyMasterModel):
  File "/home/tt/django_reproducer/lib64/python3.6/site-packages/django/db/models/base.py", line 78, in __new__
    new_class = super_new(cls, name, bases, new_attrs, **kwargs)
TypeError: __init_subclass__() missing 1 required positional argument: 'cls'

To reproduce, just use the code below in models.py:
(FWIW, I created a django project and then polls app using ​the tutorial and then added the code to models.py)

from django.db import models


# It works as a mixin class.
# Expected/actual result: MyModel.some_attr == 'added_by __init_subclass__'

class MyMixin:
    def __init_subclass__(cls, **kwargs):
         cls.some_attr = 'added_by __init_subclass__'
         super().__init_subclass__(**kwargs)

class MyModel(models.Model, MyMixin):
    pass


# It doesn't work directly in the subclass of the Model class
# Expected result: MyPluginModel.some_attr == 'added_by __init_subclass__'
# Actual result: TypeError: __init_subclass__() missing 1 required positional argument: 'cls'

class MyMasterModel(models.Model):  # remove inheritance from models.Model and it will work.
    def __init_subclass__(cls, **kwargs):
         cls.some_attr = 'added_by __init_subclass__'
         super().__init_subclass__(**kwargs)

class MyPluginModel(MyMasterModel):
    pass

OS: Fedora 28
Django: 2.1.4
Python: 3.6.6

[Tatiana Tereshchenko]

It seems that explicit @classmethod decorator helps, though ​PEP says that requiring it is among "rejected design options".
See ​example of __init_subclass__ usage from PEP

[Mariusz Felisiak]

​PR

[Tim Graham <timograham@…>]

In a68ea23:

Fixed #29970, #30041 -- Made ModelBase.new() pass attrs without contribute_to_class() to type.new().