Mocking a Custom File Storage Backend

Question

I've created a custom file storage backend that calls out to Amazon S3 using boto and stores the files there (I know django-storages handles this as well, but we ran into several issues with it). I'm storing it in a utils module and using it in my models like this:

from utils.s3 import S3Storage

class Photo(models.Model):
  image = models.ImageField(storage=S3Storage(), upload_to="images")

Thus any time a photo is created with an image file, the image file is uploaded to an S3 bucket.

I don't want to make calls out to S3 during my tests, but figuring out exactly what to mock in this situation is difficult. I can't mock out the entire image field, because I need to test creating the model through Tastypie.

Any ideas?

Answer 1

You can just mock out the _save method in S3Storage class to avoid uploading to S3. You can use FileSystemStorage instead.

My solution for your case would be like this:

import mock
from utils.s3 import S3Storage
from django.core.files.storage import FileSystemStorage


fss = FileSystemStorage()

@mock.patch.object(S3Storage, '_save', fss._save)
def test_something():
    assert True

Answer 2

I tried many other solutions, like overwriting in settings DEFAULT_FILE_STORAGE or the solution of Manh Tai. The problem of everything is that Django loads in memory all models when initialising, and this makes a bit unintuitive to modify a model attribute once it is set.

Tested with Django 2.1 and Python 3:

from unittest.mock import MagicMock
from django.core.files.storage import Storage
from django.core.files.uploadedfile import SimpleUploadedFile

class CreatePhotoTest(TestCase):
    def test_post_photo(self):
        def generate_filename(filename):
            return filename

        def save(name, content, max_length):
            return name

        storage_mock = MagicMock(spec=Storage, name='StorageMock')
        storage_mock.generate_filename = generate_filename
        storage_mock.save = MagicMock(side_effect=save)
        storage_mock.url = MagicMock(name='url')
        storage_mock.url.return_value = 'http://example.com/generated_filename.png'

        Photo._meta.get_field('image').storage = storage_mock

        img = SimpleUploadedFile('file.png', b"file_content", content_type="image/png")
        data = {
            'signed_contract': img
        }
        response = self.client.post('/endpoint', data, format='multipart')

        self.assertTrue(storage_mock.save.called)
        generated_filename = storage_mock.save.call_args_list[0][0][0]
        uploaded_file = storage_mock.save.call_args_list[0][0][1]
        self.assertEqual(uploaded_file.name, 'file.pdf')

I created generate_filename() and save(), but you don't need to do that if you do not want. This was just done to emulate as much as possible the behaviour of a real Storage and to verify it in the test.