[sonic-py-common][multi ASIC] API to get a list of frontend ports (sonic-net#5221)

* [sonic-py-common][multi ASIC] utility to get a list of frontend ports from a given list of ports

Author: smaheshm

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

@@ -135,9 +135,8 @@ def get_namespaces_from_linux():
     In a multi asic platform, each ASIC is in a Linux Namespace.
     This method returns list of all the Namespace present on the device
 
-    Note: It is preferable to use this function can be used only 
-    when the config_db is not available. 
-    When configdb is available use get_all_namespaces()
+    Note: It is preferable to use this function only when config_db is not
+    available. When configdb is available use get_all_namespaces()
 
     Returns:
         List of the namespaces present in the system
@@ -253,6 +252,17 @@ def is_port_internal(port_name, namespace=None):
     return False
 
 
+def get_external_ports(port_names, namespace=None):
+    external_ports = set()
+    ports_config = get_port_table(namespace)
+    for port in port_names:
+        if port in ports_config:
+            if (PORT_ROLE not in ports_config[port] or
+                    ports_config[port][PORT_ROLE] == EXTERNAL_PORT):
+                external_ports.add(port)
+    return external_ports
+
+
 def is_port_channel_internal(port_channel, namespace=None):
 
     if not is_multi_asic():