config: T8858: fix mutable default argument in config API methods

Four Config class methods used `default=[]` as a parameter default. The
empty list is created once at function-definition time and shared across
all calls; any mutation of the default leaks across callers. This is
mitigated in practice by `_make_path` returning `(self._level + path)`
(which creates a fresh list via `+` rather than mutating via `.append()`),
but the defensive `default=None` + explicit guard is preserved as an
additional safeguard.

Methods touched:
- return_values
- list_nodes
- return_effective_values
- list_effective_nodes

Also document the `default` parameter in each method's docstring `Args:`
section and the corresponding `Returns:` clarifications.

1 files changed, 20 insertions, 8 deletions

diff --git a/python/vyos/config.py b/python/vyos/config.py
index 827246d05..7c9771ae3 100644
--- a/python/vyos/config.py
+++ b/python/vyos/config.py
@@ -480,21 +480,24 @@ class Config(object):
         else:
             return(value)
 
-    def return_values(self, path, default=[]):
+    def return_values(self, path, default=None):
         """
         Retrieve all values of a multi-value leaf node in the running or proposed config
 
         Args:
             path (str): Configuration tree path
+            default (list): Value returned (as a copy) when the node does not exist or has no values. Defaults to an empty list.
 
         Returns:
             str list: Node values, if it has any
-            []: if node does not exist
+            default.copy(): if node does not exist or has no values
 
         Note:
             This function cannot be used outside a configuration session.
             In operational mode scripts, use ``return_effective_values``.
         """
+        if default is None:
+            default = []
         if self._session_config:
             try:
                 values = self._session_config.return_values(self._make_path(path))
@@ -508,17 +511,20 @@ class Config(object):
         else:
             return(values)
 
-    def list_nodes(self, path, default=[]):
+    def list_nodes(self, path, default=None):
         """
         Retrieve names of all children of a tag node in the running or proposed config
 
         Args:
             path (str): Configuration tree path
+            default (list): Value returned (as a copy) when the tag node does not exist or has no children. Defaults to an empty list.
 
         Returns:
-            string list: child node names
+            string list: child node names, or default.copy() if the node does not exist or has no children
 
         """
+        if default is None:
+            default = []
         if self._session_config:
             try:
                 nodes = self._session_config.list_nodes(self._make_path(path))
@@ -596,16 +602,19 @@ class Config(object):
         else:
             return(value)
 
-    def return_effective_values(self, path, default=[]):
+    def return_effective_values(self, path, default=None):
         """
         Retrieve all values of a multi-value node in a running (effective) config
 
         Args:
             path (str): Configuration tree path
+            default (list): Value returned (as a copy) when the node does not exist or has no values. Defaults to an empty list.
 
         Returns:
-            str list: A list of values
+            str list: A list of values, or default.copy() if the node does not exist or has no values
         """
+        if default is None:
+            default = []
         if self._running_config:
             try:
                 values = self._running_config.return_values(self._make_path(path))
@@ -619,16 +628,19 @@ class Config(object):
         else:
             return(values)
 
-    def list_effective_nodes(self, path, default=[]):
+    def list_effective_nodes(self, path, default=None):
         """
         Retrieve names of all children of a tag node in the running config
 
         Args:
             path (str): Configuration tree path
+            default (list): Value returned (as a copy) when the tag node does not exist or has no children. Defaults to an empty list.
 
         Returns:
-            str list: child node names
+            str list: child node names, or default.copy() if the node does not exist or has no children
         """
+        if default is None:
+            default = []
         if self._running_config:
             try:
                 nodes = self._running_config.list_nodes(self._make_path(path))