[s3] fix disabling cloudfront signing with class variable (#1334)

This change fixes setting ``cloudfront_signer=None`` as a class variable
to disable cloudfront signing (or passing a custom implementation), and
is a follow up to #1326

storages/backends/s3.py

@@ -300,7 +300,8 @@ class S3Storage(CompressStorageMixin, BaseStorage):
 
     def __init__(self, **settings):
         omitted = object()
-        self.cloudfront_signer = settings.pop("cloudfront_signer", omitted)
+        if not hasattr(self, "cloudfront_signer"):
+            self.cloudfront_signer = settings.pop("cloudfront_signer", omitted)
 
         super().__init__(**settings)
 

tests/test_s3.py

@@ -894,6 +894,12 @@ def test_cloudfront_config(self):
             storage = s3.S3Storage(cloudfront_signer=None)
             self.assertIsNone(storage.cloudfront_signer)
 
+            # allow disabling cloudfront signing in subclass
+            class Storage(s3.S3Storage):
+                cloudfront_signer = None
+
+            self.assertIsNone(Storage().cloudfront_signer)
+
         storage = s3.S3Storage(cloudfront_key_id=key_id, cloudfront_key=pem)
         self.assertIsNotNone(storage.cloudfront_signer)