Conditionally load stubs based on specific version of package installed

For django-stubs, we would like to be able to load stubs conditionally, like it's done for typeshed via https://mypy.readthedocs.io/en/latest/common_issues.html#python-version-and-system-platform-checks

See https://github.com/typeddjango/django-stubs/issues/262

I'd like to propose to add a special function is_package_installed for stub files:

if is_package_installed('Django>=3.0.*')
    class Django30Class: ...
else:
    class Django22Class: ...

which will evaluate according to the rules specified in PEP440 https://www.python.org/dev/peps/pep-0440/#version-specifiers

It was discussed in https://github.com/python/typeshed/issues/153, current workaround seems to be (from https://github.com/python/typeshed/issues/153#issuecomment-389198282):

release and install a separate stub package for each version of the library

I believe it's too complex, as one needs to maintain lots of separate branches for specific versions of the supported package. And versioning of the stubs package would then be PACKAGE_VERSION.STUBS_VERSION_INCREMENT?

リンクされている mypy のドキュメントと PEP 440 のバージョン指定子の規則から始め、続いて参照されている django-stubs と typeshed の議論を読んでください。提案されている動作は、インストール済みパッケージのバージョンに基づいてスタブ宣言を条件付きで選択するための設計です。完了には、合意された仕組みと、サポート対象のバージョンチェックに関する文書化された動作が必要です。