bpo-23378: Add an extend action to argparse (GH-13305)

Add an extend action to argparse


https://bugs.python.org/issue23378

Doc/library/argparse.rst

@@ -797,6 +797,15 @@ how the command-line arguments should be handled. The supplied actions are:
     >>> parser.parse_args(['--version'])
     PROG 2.0
 
+* ``'extend'`` - This stores a list, and extends each argument value to the
+  list.
+  Example usage::
+
+    >>> parser = argparse.ArgumentParser()
+    >>> parser.add_argument("--foo", action="extend", nargs="+", type=str)
+    >>> parser.parse_args(["--foo", "f1", "--foo", "f2", "f3", "f4"])
+    Namespace(foo=['f1', 'f2', 'f3', 'f4'])
+
 You may also specify an arbitrary action by passing an Action subclass or
 other object that implements the same interface.  The recommended way to do
 this is to extend :class:`Action`, overriding the ``__call__`` method