Calculate settings for S3Boto3StorageFile when instantiated, not imported

[brianhelba]

This allows the storages.backends.s3boto3 module to be cleanly imported before Django settings are configured. This further supports the changes from #524.

Removing S3Boto3StorageFile.buffer_size as a class variable does not affect the API surface of S3Boto3StorageFile, as buffer_size was always accessible as an instance variable.

[brianhelba]

@jschneier @jdufresne I'd welcome your feedback, as this supports #524.

[brianhelba]

Ping @jschneier @jdufresne

Is there anything I should do to support review of this PR?

[jdufresne]

The change looks good to me. Could a test be added to demonstrate the effect of the change?

[brianhelba]

In the test environment, the Django settings are always implicitly configured on-demand, since the DJANGO_SETTINGS_MODULE environment variable is set.

The proof that this PR works is that from a clean Python shell (where DJANGO_SETTINGS_MODULE is not set),

from storages.backends import s3boto3

raises

django.core.exceptions.ImproperlyConfigured: Requested setting AWS_S3_FILE_BUFFER_SIZE, but settings are not configured. You must either define the environment variable DJANGO_SETTINGS_MODULE or call settings.configure() before accessing settings.

before this change and successfully imports after this change.

I could add an additional Tox test environment where DJANGO_SETTINGS_MODULE is not set, and where the test code attempts to import all of the the modules within storages. However, that seems like a lot of additional test infrastructure to maintain. On the other hand, if everyone agrees that being able to cleanly import is an important feature (since it is a major goal of #524), maybe having a test to prevent a regression is worth it.

I'd be happy to write the extra testing if the project maintainers want it.

[jdufresne]

The following test demonstrates the change, IMO. It fails before the fix and passes aftewards.

    def test_override_file_buffer_size(self):
        f = s3boto3.S3Boto3StorageFile("foo.txt", "rb", mock.MagicMock())
        self.assertEqual(f.buffer_size, 5242880)
        with override_settings(AWS_S3_FILE_BUFFER_SIZE=1024):
            f = s3boto3.S3Boto3StorageFile("foo.txt", "rb", mock.MagicMock())
            self.assertEqual(f.buffer_size, 1024)