bpo-38584: fix a bug in argparse with whitespace-only help messages

Lib/argparse.py

@@ -529,7 +529,8 @@ class HelpFormatter(object):
         if action.help:
             help_text = self._expand_help(action)
             help_lines = self._split_lines(help_text, help_width)
-            parts.append('%*s%s\n' % (indent_first, '', help_lines[0]))
+            if help_lines:
+                parts.append('%*s%s\n' % (indent_first, '', help_lines[0]))
             for line in help_lines[1:]:
                 parts.append('%*s%s\n' % (help_position, '', line))
 

Lib/test/test_argparse.py

@@ -4688,6 +4688,15 @@ class TestOptionalsHelpVersionActions(TestCase):
             self.assertPrintHelpExit(parser, format % '--help')
             self.assertRaises(AttributeError, getattr, parser, 'format_version')
 
+    def test_whitespace_help(self):
+        for formatter_class in (argparse.ArgumentDefaultsHelpFormatter, argparse.HelpFormatter,
+                                argparse.MetavarTypeHelpFormatter,
+                                argparse.RawDescriptionHelpFormatter, argparse.RawTextHelpFormatter):
+            parser = argparse.ArgumentParser(formatter_class=formatter_class)
+            parser.add_argument('--foo', action='store_true', help=' ')
+            parser.add_argument('bar', type=float, help=' ')
+            parser.format_help()  # does not raise any error
+
 
 # ======================
 # str() and repr() tests

Misc/NEWS.d/next/Library/2020-11-01-12-27-34.bpo-38584.S6K8sF.rst

@@ -0,0 +1 @@
+Fix a minor bug in argparse with whitespace-only argument help messages