Use file storage on the local filesystem in Django unit tests

test_runner.py

import shutil
import tempfile

from django.apps import apps
from django.conf import settings
from django.core.files.storage import FileSystemStorage
from django.db.models import FileField
from django.db.models.loading import get_model, get_models
from django.test.runner import DiscoverRunner


class LocalStorageDiscoverRunner(DiscoverRunner):
    """
    Use file storage on the local filesystem in unit tests.

    Inspiration: https://www.caktusgroup.com/blog/2013/06/26/media-root-and-django-tests/
    """

    def setup_test_environment(self):
        super(TempMediaDiscoverRunner, self).setup_test_environment()

        # Keep track of original storages.
        settings._original_media_root = settings.MEDIA_ROOT
        settings._original_file_storage = settings.DEFAULT_FILE_STORAGE
        settings._original_fields_storages = {}

        # Creates a temporary directory.
        settings._temp_media_dir = tempfile.mkdtemp(dir=settings.BASE_PARENT_DIR)

        # Use the FileSystemStorage for tests.
        settings.MEDIA_ROOT = settings._temp_media_dir
        settings.DEFAULT_FILE_STORAGE = 'django.core.files.storage.FileSystemStorage'

        # Use FileSystemStorage for all model fields. This is necessary because some fields will have 
        # already been built using the original file system backend by the time this code runs.
        for model in apps.get_models():
            fields = [f for f in model._meta.fields if isinstance(f, FileField)]
            for field in fields:
                model_path = '%s.%s' % (model._meta.app_label, model._meta.model_name)
                original_storage = (field.name, field.storage)
                original_storages = settings._original_fields_storages.setdefault(model_path, [])
                original_storages.append(original_storage)
                field.storage = FileSystemStorage(location=settings.MEDIA_ROOT)

    def teardown_test_environment(self):
        super(TempMediaDiscoverRunner, self).teardown_test_environment()

        # Delete the temporary directory.
        shutil.rmtree(settings.MEDIA_ROOT, ignore_errors=True)

        # Restore original storage.
        settings.MEDIA_ROOT = settings._original_media_root
        settings.DEFAULT_FILE_STORAGE = settings._original_file_storage

        # Restore original storages for all model fields.
        for model_path, original_storages in settings._original_fields_storages.items():
            model = apps.get_model(*model_path.split('.'))
            for field_name, original_storage in original_storages:
                field = model._meta.get_field(field_name)
                field.storage = original_storage

        del settings._original_media_root
        del settings._original_file_storage
        del settings._original_fields_storages

Put test_runner.py in your main project directory, then add the following to your testing settings:

# Use a test runner that switches the file storage backend to django's default of FileSystemStorage.
TEST_RUNNER = '<< project_name >>.test_runner.TempMediaDiscoverRunner'