deleting a file using post_delete

Using Django Using the ORM
1

I tried to make sure a resource is deleted, when the corresponding instance is deleted:

my model:

class SomeField(models.Model):
    name = models.CharField("name", max_length = 32, unique = True)
    
class SomeResource(models.Model):
    somefield = models.ForeignKey(SomeField, on_delete = models.CASCADE)
    file = models.FileField("the file", max_length = 255)

first approach:

def file_cleanup(sender, **kwargs):
    if Path(kwargs["instance"].file.path).is_file():
        kwargs["instance"].file.delete()

signal:

post_delete.connect(file_cleanup, sender=SomeResource, dispatch_uid="someresource.file_cleanup")

But in the database I see the instance is never deleted. The file is correctly removed. If I comment out the kwargs["instance"].file.delete() line, the instance is correctly removed. Why?

my workaround:

def file_cleanup(sender, **kwargs):
    path = Path(kwargs["instance"].file.path)
    if path.is_file():
        path.unlink()

I am trying to understand why the first “solution” did not work.

2

What happens if you change this from a post_delete sitgnal to a pre_delete signal?

From the docs for the post_delete signal:

instance

The actual instance being deleted.

Note that the object will no longer be in the database, so be very careful what you do with this instance.

(Note: I have no actual idea what is happening here, or what the effects may be of what you’re doing - I learned a long time ago to avoid signals except in those cases where they’re actually needed, and so I’ve never encountered anything like this.)

3

The delete method (you are calling) on FieldFile accepts a save parameter which defaults to True. So, in your example, the model object is saved after deleting file, thus recreating the object in database. If you pass save=False parameter when calling delete, the model object should not be recreated in database.

1 Like
4

Side note: when checking for existence of file, using

you suppose the storage backend is a FileSystemStorage.

If you want your code to be agnostic from backend storage (for example if you change it to S3Storage from django-storages) and still want to have your cleaning function working for such backend, you could use following test :

if kwargs["instance"].file.storage.exists(kwargs["instance"].file.name):
5

When trying your suggestion I end up with

TypeError file_cleanup() missing 1 required positional argument: 'sender'

def file_cleanup(sender, **kwargs):
    from .models import NewLayoutResource
    if isinstance(sender, NewLayoutResource):
        if kwargs["instance"].file.storage.exists(kwargs["instance"].file.name):
            kwargs["instance"].file.delete(save = False)

I thought using signals was smart, but after reading @KenWhitesell 's post I decided to go for:

class SomeResource(models.Model):
    somefield = models.ForeignKey(SomeField, on_delete = models.CASCADE)
    file = models.FileField("the file", max_length = 255)

    def delete_file(self):
        if (p := Path(self.file.path)).is_file():
            p.unlink()

and in the clean method of my SomeResourceForm:

def clean(self):
    clean_data = super().clean()
    if (self.has_changed() and "file" in self.initial.keys()) or clean_data["DELETE"] == True:
        self.instance.delete_file()
        return clean_data

which also works just fine and avoids all signals :slight_smile: I would probably need some time to wrap my head around pre/post_delete signals :frowning: