Add common functions applicable to single/multi asic platforms (#5224)

* Add common functions applicable to single/multi asic platforms
* Raise exception if invalid namespace is given as input.

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

@@ -123,11 +123,17 @@ def is_multi_asic():
 
 
 def get_asic_id_from_name(asic_name):
+    """
+    Get the asic id from the asic name for multi-asic platforms
+    In single ASIC platforms, it would fail and throw an exception.
 
+    Returns:
+        asic id.
+    """
     if asic_name.startswith(ASIC_NAME_PREFIX):
         return asic_name[len(ASIC_NAME_PREFIX):]
     else:
-        return None
+        raise ValueError('Unknown asic namespace name {}'.format(asic_name))
 
 # Get the ASIC id from the asic.conf file.
 def get_asic_device_id(asic_id):
@@ -336,3 +342,33 @@ def is_bgp_session_internal(bgp_neigh_ip, namespace=None):
         else:
             return False
     return False
+
+def get_front_end_namespaces():
+    """
+    Get the namespaces in the platform. For multi-asic devices we get the namespaces
+    mapped to asic which have front-panel interfaces. For single ASIC device it is the
+    DEFAULT_NAMESPACE which maps to the linux host.
+
+    Returns:
+        a list of namespaces
+    """
+    namespaces = [DEFAULT_NAMESPACE]
+    if is_multi_asic():
+        ns_list = get_all_namespaces()
+        namespaces = ns_list['front_ns']
+
+    return namespaces
+
+
+def get_asic_index_from_namespace(namespace):
+    """
+    Get asic index from the namespace name.
+    With single ASIC platform, return asic_index 0, which is mapped to the only asic present.
+
+    Returns:
+        asic_index as an integer.
+    """
+    if is_multi_asic():
+        return int(get_asic_id_from_name(namespace))
+
+    return 0