Document overriding STORAGES for tests when using callable storage in a FileField

[emillumine]

It seems that the default storage and other user defined storages dont have the same behaviour in tests.
When overriding the settings.STORAGES in tests we can easily replace the default storage whereas it doesn't work for another user defined storage.
I tried to reproduce my problem with a minimal code below.

It may seems successful at first because the first assertion assert storages["other_storage"]._location == "other_location" is green. But the last one assert test.other_file.storage._location == "other_location" is not. Its value is the one defined in settings.STORAGES.

I would expect a similar behaviour in both cases.

# settings.py
STORAGES = {
    "default": {
        "BACKEND": "django.core.files.storage.FileSystemStorage",
    },
    "staticfiles": {
        "BACKEND": "django.contrib.staticfiles.storage.StaticFilesStorage",
    },
    "other_storage": {
        "BACKEND": "django.core.files.storage.FileSystemStorage",
    }
}

# test.py

from django.test import override_settings
from django.core.files.storage import default_storage
from django.core.files.storage import storages
from django.core.files.storage import InMemoryStorage
from django.db import models
from django.core.files.base import ContentFile


def select_other_storage():
    return storages["other_storage"]

def select_default_storage():
    return default_storage


class TestModel(models.Model):
    other_file = models.FileField(storage=select_other_storage)
    default_file = models.FileField(storage=select_default_storage)

    class Meta:
        app_label = "test"

@override_settings(
    STORAGES={
        "default": {
            "BACKEND": "django.core.files.storage.InMemoryStorage",
            "OPTIONS": {
                "location": "default_location",
                },
            },
        "other_storage": {
            "BACKEND": "django.core.files.storage.InMemoryStorage",
            "OPTIONS": {
                "location": "other_location",
                },
        }
    }
)
def test_override_settings():
    assert default_storage._location == "default_location"
    assert storages["other_storage"]._location == "other_location"

    test = TestModel(
        other_file=ContentFile("test data", name="test.pdf"),
        default_file=ContentFile("test data", name="test.pdf"),
    )
    assert isinstance(test.default_file.storage, InMemoryStorage)
    assert test.default_file.storage._location == "default_location"
    # following assertions fail
    assert isinstance(test.other_file.storage, InMemoryStorage)
    assert test.other_file.storage._location == "other_location"

[Sarah Boyce]

To me, this is expected based off the documentation:

​https://docs.djangoproject.com/en/5.1/topics/files/#using-a-callable

Your callable will be evaluated when your models classes are loaded...

However, I think it might be nice to update the docs with an example similar to the default storage in order to make this easier to test.

We would need to check if this works but maybe something like:

from django.core.files.storage import storages
from django.db import models
from django.utils.functional import LazyObject


class MyStorage(LazyObject):
    def _setup(self):
        self._wrapped = storages["mystorage"]


select_storage = MyStorage()

class MyModel(models.Model):
    upload = models.FileField(storage=select_storage)

With a comment around testing and why you might want to do this