diff -r c3539eb02470 Lib/argparse.py
--- a/Lib/argparse.py	Tue Sep 11 15:47:28 2012 +0200
+++ b/Lib/argparse.py	Tue Sep 11 11:16:16 2012 -0400
@@ -1961,6 +1961,7 @@
                     # twice (which may fail) if the argument was given, but
                     # only if it was defined already in the namespace
                     if (action.default is not None and
+                        isinstance(action, _StoreAction) and
                         hasattr(namespace, action.dest) and
                         action.default is getattr(namespace, action.dest)):
                         setattr(namespace, action.dest,
diff -r c3539eb02470 Lib/test/test_argparse.py
--- a/Lib/test/test_argparse.py	Tue Sep 11 15:47:28 2012 +0200
+++ b/Lib/test/test_argparse.py	Tue Sep 11 11:16:16 2012 -0400
@@ -4607,6 +4607,17 @@
         args = parser.parse_args([])
         self.assertEqual(NS(foo='foo_converted'), args)
 
+    def test_issue_15906(self):
+        # Issue #15906: When action='append', type=str, default=[] are
+        # providing, the dest value was the string representation "[]" when it
+        # should have been an empty list.
+        parser = argparse.ArgumentParser()
+        parser.add_argument('--test', dest='test', type=str,
+                            default=[], action='append')
+        args = parser.parse_args()
+        self.assertEqual(args.test, [])
+
+
 # ======================
 # parse_known_args tests
 # ======================