fix: accept lists for custom actions with nargs

configargparse.py

@@ -1472,17 +1472,15 @@ def convert_item_to_command_line_arg(self, action, key, value):
                     "'false', 'yes', 'no', 'on', 'off', '1' or '0'" % (key, value)
                 )
         elif isinstance(value, list):
-            accepts_list_and_has_nargs = (
-                action is not None
-                and action.nargs is not None
-                and (
-                    isinstance(action, argparse._StoreAction)
-                    or isinstance(action, argparse._AppendAction)
-                )
-                and (
-                    action.nargs in ("+", "*")
-                    or (isinstance(action.nargs, int) and action.nargs > 1)
-                )
+            # Only nargs is relevant here: any action, including a custom
+            # argparse.Action subclass, consumes several values when nargs
+            # allows it. Testing the action class as well would be redundant,
+            # since actions that cannot consume a list have an nargs that
+            # fails the test below anyway: 0 or None for store_const, count
+            # and friends, 'A...' for subparsers, '...' for REMAINDER.
+            accepts_list_and_has_nargs = action is not None and (
+                action.nargs in ("+", "*")
+                or (isinstance(action.nargs, int) and action.nargs > 1)
             )
 
             if action is None or isinstance(action, argparse._AppendAction):

tests/test_configargparse.py

@@ -430,6 +430,32 @@ def testPositionalAndConfigVarLists(self):
         self.assertEqual(ns.arg, ["Shell", "someword", "anotherword"])
         self.assertEqual(ns.a, "positional_value")
 
+    def testCustomActionWithNargsAndConfigVarList(self):
+        # https://github.com/bw2/ConfigArgParse/issues/354
+        class CustomAction(configargparse.Action):
+            def __call__(self, parser, namespace, values, option_string=None):
+                setattr(namespace, self.dest, values)
+
+        self.initParser()
+        self.add_arg("-x", "--arg", nargs="+", action=CustomAction)
+
+        ns = self.parse("", config_file_contents="""arg = [foo, bar]""")
+        self.assertEqual(ns.arg, ["foo", "bar"])
+
+    def testCustomActionWithoutNargsAndConfigVarList(self):
+        class CustomAction(configargparse.Action):
+            def __call__(self, parser, namespace, values, option_string=None):
+                setattr(namespace, self.dest, values)
+
+        self.initParser()
+        self.add_arg("-x", "--arg", action=CustomAction)
+
+        self.assertParseArgsRaises(
+            "arg can't be set to a list",
+            args="",
+            config_file_contents="""arg = [foo, bar]""",
+        )
+
     def testMutuallyExclusiveArgs(self):
         config_file = tempfile.NamedTemporaryFile(mode="w", delete=False)