#9444: use first of prefix_chars for help opt instead of raising error

An argparse option parser created with a prefix_chars that did not
include a '-' would happily add -h and --help options, and then throw
an error when it tried to format the help because the - was an invalid
prefix character.  This patch makes it use the first character of
prefix_chars as the character for the help options if and only if '-'
is not one of the valid prefix_chars.

Fix by Theodore Turocy, unit tests by Catherine Devlin.

1 files changed, 5 insertions, 2 deletions

diff --git a/Lib/argparse.py b/Lib/argparse.py
index c12a16d..d556621 100644
--- a/Lib/argparse.py
+++ b/Lib/argparse.py
@@ -1561,13 +1561,16 @@ class ArgumentParser(_AttributeHolder, _ActionsContainer):
 
         # add help and version arguments if necessary
         # (using explicit default to override global argument_default)
+        default_prefix = '-' if '-' in prefix_chars else prefix_chars[0]
         if self.add_help:
             self.add_argument(
-                '-h', '--help', action='help', default=SUPPRESS,
+                default_prefix+'h', default_prefix*2+'help',
+                action='help', default=SUPPRESS,
                 help=_('show this help message and exit'))
         if self.version:
             self.add_argument(
-                '-v', '--version', action='version', default=SUPPRESS,
+                default_prefix+'v', default_prefix*2+'version',
+                action='version', default=SUPPRESS,
                 version=self.version,
                 help=_("show program's version number and exit"))