bpo-38843: Document behavior of default when the attribute is already set (GH-23653)
diff --git a/Doc/library/argparse.rst b/Doc/library/argparse.rst
index a32b999..02cd70f 100644
--- a/Doc/library/argparse.rst
+++ b/Doc/library/argparse.rst
@@ -696,7 +696,7 @@
* const_ - A constant value required by some action_ and nargs_ selections.
* default_ - The value produced if the argument is absent from the
- command line.
+ command line and if it is absent from the namespace object.
* type_ - The type to which the command-line argument should be converted.
@@ -1006,6 +1006,14 @@
>>> parser.parse_args([])
Namespace(foo=42)
+If the target namespace already has an attribute set, the action *default*
+will not over write it::
+
+ >>> parser = argparse.ArgumentParser()
+ >>> parser.add_argument('--foo', default=42)
+ >>> parser.parse_args([], namespace=argparse.Namespace(foo=101))
+ Namespace(foo=101)
+
If the ``default`` value is a string, the parser parses the value as if it
were a command-line argument. In particular, the parser applies any type_
conversion argument, if provided, before setting the attribute on the