Default param is None rather than []

common/lib/capa/capa/xqueue_interface.py

@@ -65,7 +65,7 @@ class XQueueInterface(object):
         self.auth = django_auth
         self.session = requests.session(auth=requests_auth)
         
-    def send_to_queue(self, header, body, files_to_upload=[]):
+    def send_to_queue(self, header, body, files_to_upload=None):
         '''
         Submit a request to xqueue.
         
@@ -98,6 +98,7 @@ class XQueueInterface(object):
         payload = {'xqueue_header': header,
                    'xqueue_body'  : body}
         files = {} 
+        if files_to_upload is not None:
             for f in files_to_upload:
                 files.update({ f.name: f })