From 617616e1c5254635eafefba3316437f84df5cbef Mon Sep 17 00:00:00 2001
From: Lucas Cimon <925560+Lucas-C@users.noreply.github.com>
Date: Sun, 1 Nov 2020 12:27:29 +0100
Subject: [PATCH] bpo-38584: fix a bug in argparse with whitespace-only help
 messages

---
 Lib/argparse.py                                          | 3 ++-
 Lib/test/test_argparse.py                                | 9 +++++++++
 .../Library/2020-11-01-12-27-34.bpo-38584.S6K8sF.rst     | 1 +
 3 files changed, 12 insertions(+), 1 deletion(-)
 create mode 100644 Misc/NEWS.d/next/Library/2020-11-01-12-27-34.bpo-38584.S6K8sF.rst

diff --git a/Lib/argparse.py b/Lib/argparse.py
index 2fb1da59f94..f6cb7129e14 100644
--- a/Lib/argparse.py
+++ b/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))
 
diff --git a/Lib/test/test_argparse.py b/Lib/test/test_argparse.py
index e98c15b11af..1ea02998eec 100644
--- a/Lib/test/test_argparse.py
+++ b/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
diff --git a/Misc/NEWS.d/next/Library/2020-11-01-12-27-34.bpo-38584.S6K8sF.rst b/Misc/NEWS.d/next/Library/2020-11-01-12-27-34.bpo-38584.S6K8sF.rst
new file mode 100644
index 00000000000..6b1206fd790
--- /dev/null
+++ b/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