Adding comment about using _connection in storage.blob.

Doing this to address review commits. Also testing current()
in the presence of an empty batch stack.

gcloud/storage/blob.py

@@ -291,6 +291,12 @@ def download_to_file(self, file_obj, client=None):
             headers['Range'] = 'bytes=0-%d' % (self.chunk_size - 1,)
         request = http_wrapper.Request(download_url, 'GET', headers)
 
+        # Use the private ``_connection`` rather than the public
+        # ``.connection``, since the public connection may be a batch. A
+        # batch wraps a client's connection, but does not store the `http`
+        # object. The rest (API_BASE_URL and build_api_url) are also defined
+        # on the Batch class, but we just use the wrapped connection since
+        # it has all three (http, API_BASE_URL and build_api_url).
         download.InitializeDownload(request, client._connection.http)
 
         # Should we be passing callbacks through from caller?  We can't

gcloud/storage/test_batch.py

@@ -90,8 +90,11 @@ def test_current(self):
         credentials = _Credentials()
         client = Client(project=project, credentials=credentials)
         batch1 = self._makeOne(client)
+        self.assertTrue(batch1.current() is None)
+
         client._push_batch(batch1)
         self.assertTrue(batch1.current() is batch1)
+
         batch2 = self._makeOne(client)
         client._push_batch(batch2)
         self.assertTrue(batch1.current() is batch2)
@@ -372,11 +375,10 @@ def test_as_context_mgr_wo_error(self):
         expected = _Response()
         expected['content-type'] = 'multipart/mixed; boundary="DEADBEEF="'
         http = _HTTP((expected, _THREE_PART_MIME_RESPONSE))
-        connection = _Connection(http=http)
         project = 'PROJECT'
         credentials = _Credentials()
         client = Client(project=project, credentials=credentials)
-        client._connection = connection
+        client._connection._http = http
 
         self.assertEqual(list(client._batch_stack), [])