pythongh-124745: Optimize argparse actions 'append', 'append_const' and 'extend'

Previously, the target list was copied every time the action was taken.
Now only the default value is copied the first time the action is taken.

Author: serhiy-storchaka

Lib/argparse.py

@@ -1016,7 +1016,8 @@ def __init__(self,
 
     def __call__(self, parser, namespace, values, option_string=None):
         items = getattr(namespace, self.dest, None)
-        items = _copy_items(items)
+        if items is self.default:
+            items = _copy_items(items)
         items.append(values)
         setattr(namespace, self.dest, items)
 
@@ -1045,7 +1046,8 @@ def __init__(self,
 
     def __call__(self, parser, namespace, values, option_string=None):
         items = getattr(namespace, self.dest, None)
-        items = _copy_items(items)
+        if items is self.default:
+            items = _copy_items(items)
         items.append(self.const)
         setattr(namespace, self.dest, items)
 
@@ -1238,7 +1240,8 @@ def __call__(self, parser, namespace, values, option_string=None):
 class _ExtendAction(_AppendAction):
     def __call__(self, parser, namespace, values, option_string=None):
         items = getattr(namespace, self.dest, None)
-        items = _copy_items(items)
+        if items is self.default:
+            items = _copy_items(items)
         items.extend(values)
         setattr(namespace, self.dest, items)
 

Misc/NEWS.d/next/Library/2024-10-02-21-34-08.gh-issue-124745.efyaxu.rst

@@ -0,0 +1,4 @@
+Optimize :mod:`argparse` actions 'append', 'append_const' and 'extend' for
+the case when the option is specified many times.
+The targed list no longer copied every time the action is taken, only
+the default value is copied the first time the action is taken.