Mypy flags `unreachable` on `pre-save` and checking for instance ID

[sshishov]

Bug report

We are implementing pre-save of some models and there usually we check if the instance is being created or not by doing

if instance.id is None:
    ...  # creating the instance logic

What's wrong

import typing as t

from django.db import models as django_models
from django import dispatch as django_dispatch

@django_dispatch.receiver(django_models.signals.pre_save, sender=my_app.MyModel)
def project_pre_save(sender: type[my_app.MyModel], instance: my_app.MyModel, **kwargs: t.Any) -> None:
    if instance.id is None:
        add_something_to_instance(instance)  # <-- Statement is unreachable  [unreachable]
    ...

In this case mypy flagging this with the following error indicated above

How is that should be

On pre-save instance.id should be either the Field or None as it can be None in case of creation of the object.
Do not know how to properly implement it though...

System information

• OS: MacOS / Linux
• python version: 3.11.5
• django version: 3.2.23
• mypy version: 1.9.0
• django-stubs version: 4.2.7
• django-stubs-ext version: 4.2.7

[sobolevn]

Yes, this is the trade-off we had to make. We can add fake UnsavedModel subclass (to stubs and ext) of Model with id set to T | None

What do you think?

[sshishov]

Hi @sobolevn

Thank you for the report, I just have a question how this UnsavedModel will be used by people. If it will require a lot of "hand-writing" or workarounds then better to keep it as is and use type: ignore[<specific-error>] instead.

Will it be like a wrapper? Like we have WithAnnotations for queryset/model?

If it will be like a wrapper, then I am "supporting" this approach!

Thanks for your valuable time! ⏲️