Replaced the block of logic in back-end LAG determination by using on…

…e single list expansion processing in get_back_end_interface_set()

src/sonic-py-common/sonic_py_common/multi_asic.py

@@ -337,12 +337,13 @@ def get_back_end_interface_set(namespace=None):
     for ns in ns_list:
         config_db = connect_config_db_for_ns(ns)
         port_channels = config_db.get_table(PORT_CHANNEL_CFG_DB_TABLE)
-        for port_channel, lag_info in port_channels.items():
-            if 'members' in lag_info:
-                members = lag_info['members']
-                if members[0] in bk_end_intf_list:
-                    bk_end_intf_list.append(port_channel)
-
+        # a back-end LAG must be configured with all of its member from back-end interfaces.
+        # mixing back-end and front-end interfaces is miss configuration and not allowed.
+        # To determine if a LAG is back-end LAG, just need to check its first member is back-end or not
+        # is sufficient. Note that a user defined LAG may have empty members so the list expansion logic
+        # need to ensure there are members before inspecting member[0].
+        bk_end_intf_list.extend([port_channel for port_channel, lag_info in port_channels.items()\
+                                if 'members' in lag_info and lag_info['members'][0] in bk_end_intf_list])
     a = set()
     a.update(bk_end_intf_list)
     return a