fix s3 delete_many (#38)

* fix s3 delete_many

Couldn't find any documentation around this issue, but I empirically discovered that `delete_objects` with an empty array causes the request to fail. Also, this is a valid optimization anyway.

* add tests and comments

* add comment

* use mock

* install mock unconditionally

flask_annex/s3.py

@@ -40,11 +40,16 @@ def delete(self, key):
         self._client.delete_object(Bucket=self._bucket_name, Key=key)
 
     def delete_many(self, keys):
+        # casting to tuple because keys might be iterable
+        objects = tuple({'Key': key} for key in keys)
+        if not objects:
+            # this is not just an optimization, boto fails if the array
+            # is empty
+            return
+
         self._client.delete_objects(
             Bucket=self._bucket_name,
-            Delete={
-                'Objects': tuple({'Key': key} for key in keys),
-            },
+            Delete={'Objects': objects},
         )
 
     def get_file(self, key, out_file):

tests/test_s3.py

@@ -2,6 +2,7 @@
 from io import BytesIO
 import json
 
+from mock import Mock
 import pytest
 
 from flask_annex import Annex
@@ -107,6 +108,14 @@ def test_get_upload_info_max_content_length(self, app, client):
     def assert_app_config_content_length_range(self, conditions):
         assert get_condition(conditions, 'content-length-range') == [0, 100]
 
+    def test_delete_many_empty_list(self, annex, monkeypatch):
+        mock = Mock()
+        monkeypatch.setattr(annex._client, 'delete_objects', mock)
+
+        annex.delete_many(tuple())
+
+        mock.assert_not_called()
+
 
 class TestS3AnnexFromEnv(TestS3Annex):
     @pytest.fixture

tox.ini

@@ -6,6 +6,7 @@ usedevelop = True
 
 deps =
     flake8
+    mock
     pytest
     pytest-cov