Custom storage backend's not entirely decoupled from FileField

[Korijn van Golen]

Let me start by saying that I've implemented custom FileFields that can handle Numpy objects, to provide some convenience methods to access the typed objects.

Later, when implementing a custom storage backend for Azure Storage, I encountered some issues when handling filenames. Cloud storage doesn't behave like local file system storage does. Most of Django's code is agnostic of this problem, except for a small section of FileField:

​https://github.com/django/django/blob/77b8d8cb6d6d6345f479c68c4892291c1492ba7e/django/db/models/fields/files.py#L306-L320

In Azure, blobs are stored in containers, and are optionally stored in subfolders (known as prefixes in Azure-speak). To be able to use the upload_to property, I need to be able to pass the full path, e.g.:

container/blob
container/sub/blob
container/sub/sub/blob

Unfortunately, FileField strips the directory part and only passes the blob name to my custom storage backend's get_valid_name implementation, where I really need to validate the whole string. I ended up subclassing FileField:

class CloudStorageFileField(FileField):
    """
    Provides some overrides to enable cloud storage backends
    """

    def get_directory_name(self):
        return force_text(datetime.datetime.now().strftime(force_str(self.upload_to)))

    def get_filename(self, filename):
        return filename

    def generate_filename(self, instance, filename):
        if callable(self.upload_to):
            filename = self.upload_to(instance, filename)
            filename = self.storage.get_valid_name(filename)
            return filename

        return self.get_directory_name() + self.storage.get_valid_name(filename)

As you can see, all I did was remove the local file system related logic. Unfortunately, my custom file fields need to inherit from this class to work with Azure storage, and when I switch to local storage in a different environment, they have to inherit from the regular FileField! This is obviously an undesirable situation, imposed by the otherwise-great tight coupling to local file system logic in the three methods of FileField.

In order to truly decouple this, FielField would need to be a little bit more agnostic about the storage backend. This could be done by moving the generate_filename method to the backend entirely, for example.

I classified this as a bug, as this case shows that you're not "entirely" able to do anything you want when implementing custom storage backends.

You can see the full implementation of both the backend and the numpyfilefield here: ​https://gist.github.com/Korijn/e0bcbdcedb494509973a

Your thoughts?