FileField.__init__ is triggering storage LazyObject resolution at boot time

[Fabien MICHEL]

There is this code in FileField.__init__ :

self.storage = storage or default_storage

The problem is that if we want to have a LazyObject as storage for the field, it is still resolved at boot time, and it seems unecessary to trigger resolution at this moment. This seems not expected as the documentation itself suggest to use LazyObject in case we require a callable to be overridable during tests : ​https://docs.djangoproject.com/en/5.2/topics/files/#using-a-callable

Using storage or default_storage is calling storage.__bool__ which trigger resolution on default LazyObject.

A solution would be to change this line by :

self.storage = storage if storage is not None else default_storage

A workaround for the case specified in the documentation is to override __bool__ on LazyObject so it always returns True.

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

    def __bool__(self):
        return True

[Sarah Boyce]

Related #36504

[Sarah Boyce]

Thank you for the ticket! I have tested and I agree with this approach.
Given there were some efforts started in #36504, I have added a comment there and marking this as a duplicate to try and prevent contradictory/overlapping effort

[Clifford Gama]

Reopening because this issue can be fixed independently of #36504--it neither contradicts nor, strictly speaking, overlap. The reason being that for override_settings to work on custom storages (#36504), we still need to write a custom signal receiver. The solution proposed here only seems to solve #36504 because we're resolving the lazy object for the first and only time after calling override_settings. However, whatever value we get with the first access will persist and cannot be overridden again.

The following modification to the docs example should demonstrate:

def test_storage(self):
    model = MyModel()
    self.assertIsInstance(model.upload.storage, FileSystemStorage)  # Assuming this is in settings
    with override_settings(
        STORAGES={
            "mystorage": {
                "BACKEND": "django.core.files.storage.InMemoryStorage",
            }
        }
     ):
         self.assertIsInstance(model.upload.storage, InMemoryStorage)

[Natalia Bidart]

Thank you Clifford for your detailed explanation.

[Natalia Bidart]

Fixed in main in 6862d46dd96d71d80d6d2fa9873a93d811b39562.
Fixed in [6.0.x] in 5c114ce2d8479549ee9dd486bc28df5b2e3d54ad.