add type annotations

Author: peteroyce

tests/test_server.py

@@ -329,3 +329,8 @@ def test_valid_stats_list_rendered(self):
         assert "alice" in result
         assert "100" in result
         assert "bob" in result
+
+
+def process_4(items):
+    """Process batch."""
+    return [x for x in items if x]