Add __class_getitem__ method to managers and querysets

[Maxim Kurnikov]

I'm working on a pep561 stubs package for Django here
​https://github.com/mkurnikov/django-stubs

In order to be able to fully support managers as a generics, one needs to write

from django.db import models

class UserManager(models.Manager[User]):
    def get_or_404(self) -> User:
        pass

class User(models.Model):
    objects = UserManager()

but it fails at models.Manager[User] .

There's a new method __class_getitem__ in python3.7 to get around that, defined in pep560
​https://www.python.org/dev/peps/pep-0560/

[Greg W]

Is this the error you are getting:

NameError: name 'User' is not defined

If not, can you please let me know what the error is?

[Maxim Kurnikov]

TypeError: 'type' object is not subscriptable

Look at
​https://www.python.org/dev/peps/pep-0560/#class-getitem

and how it was implemented before with GenericMeta
​https://www.python.org/dev/peps/pep-0484/#user-defined-generic-types

Minimal repro should be this one

from django.db import models

class UserManager(models.Manager['User']):
    pass

class User(models.Model):
    objects = UserManager()

[Greg W]

I got __class_getitem__ implemented. However, I am having an issue with models.Manager['User']

Isn't 'User' just a string which __class_getitem__ would not be valid on? I think the idea is to index the class User, but this class has not been defined prior to Manager[User].

Let me know if I am off base.

[Carlton Gibson]

Hi Maxim.

I'm going to close this as needsinfo right now. Not because I'm against it but because it's part of the wider discussion around Type Hinting in Django.

I see you posted to the Django-Developers thread about this: ​https://groups.google.com/d/topic/django-developers/trTEbURFhEY/discussion

Can I ask you to add your effort there? I think there's a group of people who are there or there-abouts with this: as ever, it just needs a bit of a nudge.
(There was talk of a DEP, of a working groups, of... — we just need to get there.)

Thanks.

I'll post there to try and re-liven it.

[Tim Graham]

(And #29299 is the Someday/Maybe ticket for adding type hints and variable annotations.)