Unable to use inlineformset_factory's returned class as baseclass for another class

[maziar-dandc]

Bug report

What's wrong

Cannot use the class returned from inlineformset_factory as a baseclase for creating another class, failing test:

- case: inlineformset_factory_extended
  main: |
    from typing import Any, Type
    from django import forms
    from myapp.models import Article, Category
    ArticleFS: Type[forms.BaseInlineFormSet[Article, Category, Any]] = forms.inlineformset_factory(Category, Article)
    ArticleFS(instance=Article())  # E: Argument "instance" to "BaseInlineFormSet" has incompatible type "Article"; expected "Optional[Category]"
    class CustomArticleFS(ArticleFS): # It will throw a "ArticleFS" is not a valid type error here.
      def __init__(self, *args, **kwargs):
        print('hi')
        super().__init__(*args, **kwargs)
    fs = CustomArticleFS(instance=Category())
    reveal_type(fs.instance)  # N: Revealed type is "myapp.models.Category"
  installed_apps:
    - myapp
  files:
    - path: myapp/__init__.py
    - path: myapp/models.py
      content: |
        from django.db import models

        class Article(models.Model):
            pass
        class Category(models.Model):
            pass

@sterliakov has provided some very helpful context here in this comment
#976 (comment)
related issue on mypy linked y @sterliakov: python/mypy#2813
related question on stackoverflow: https://stackoverflow.com/questions/59634937/variable-foo-class-is-not-valid-as-type-but-why

How is that should be

Should be able to use a dynamic class as baseclass for creating new classes, I guess as long as that baseclass's type is known.

System information

• OS:
• python version: 3.8
• django version: used django-stubs environment at this commit hash
• mypy version: 0.961
• django-stubs version: 1.12.0
• django-stubs-ext version: 0.4.3

[MaziyarMK]

python/mypy#2477

[sobolevn]

If anyone wants to write a custom get_dynamic_class_hook() plugin hook for this, I am open to accept a PR :)

[maziar-dandc]

that's my personal account, found a link and thought I'd add it here for reference.