cached_property breaks type checking

[dms-cat]

To reproduce:

1. Use a strict mypy configuration:

   [mypy]
   check_untyped_defs = true
   disallow_untyped_defs = true
   ignore_missing_imports = true
   no_implicit_optional = true
   warn_redundant_casts = true
   warn_return_any = true
   warn_unused_ignores = true
   

2. Create a model method which returns an optional instance:

   @property
   def area(self) -> Optional[Area]:
   

3. Try to use this elsewhere as though it's not optional: instance.area.code
4. Verify that mypy catches this issue:

   error: Item "None" of "Optional[Area]" has no attribute "code"
   

5. Change the @property annotation to @cached_property.

What happens:

mypy stops complaining about the broken typing.

What should happen:

mypy should still complain like above.

[Mariusz Felisiak]

Please provide a minimal sample project to reproduce this issue, currently it is hard for me to decide where is an issue (if any). Remember about minimal mypy configuration, most of provided custom configuration is probably unnecessary.

[dms-cat]

Replying to felixxm:

Please provide a minimal sample project to reproduce this issue, currently it is hard for me to decide where is an issue (if any).

To reproduce (not showing stdout/stderr):

$ cd "$(mktemp --directory)"
$ python -m venv virtualenv
$ . virtualenv/bin/activate
$ pip install Django==2.2.2 mypy==0.701
$ django-admin startproject mysite
$ cat > setup.cfg <<EOF
> [mypy]
> check_untyped_defs = true
> disallow_untyped_defs = true
> ignore_missing_imports = true
> no_implicit_optional = true
> warn_redundant_casts = true
> warn_return_any = true
> warn_unused_ignores = true
> EOF
cat > mysite/mysite/models.py <<EOF
from typing import Optional

from django.db import models
from django.utils import cached_property


class Area(models.Model):
    pass


class Site(models.Model):
    area = models.ForeignKey(Area, on_delete=models.CASCADE)

    @cached_property
    def the_area(self) -> Optional[Area]:
        area: Area = self.area
        return area


area = Area.objects.create()
site = Site(area)
print(site.the_area.code)
EOF
$ mypy mysite/mysite/models.py

The last command reports nothing. If you change @cached_property to @property it correctly reports 'mysite/mysite/models.py:22: error: Item "None" of "Optional[Area]" has no attribute "code"'

Remember about minimal mypy configuration, most of provided custom configuration is probably unnecessary.

This isn't provided configuration, we arrived at this configuration in the project as the level of validation we're comfortable with.

[Mariusz Felisiak]

Thanks for info. Custom mypy configuration is not necessary to reproduce this issue, it behaves the same without setup.cfg.

I was able to reproduce this behavior, however I don't think that it is a bug in Django. We can consider to re-open this ticket if you will be able to confirm that it is an issue in Django e.g. by providing a patch.