bpo-44986: Fixed bug with date formats like %Y-%m-%d in help messages of argparse

Lib/argparse.py

@@ -600,7 +600,11 @@ def _expand_help(self, action):
         if params.get('choices') is not None:
             choices_str = ', '.join([str(c) for c in params['choices']])
             params['choices'] = choices_str
-        return self._get_help_string(action) % params
+        expanded_help = self._get_help_string(action)
+        for param, value in params.items():
+            if f'%({param})s' in expanded_help:
+                expanded_help = expanded_help.replace(f'%({param})s', str(value))
+        return expanded_help.replace('%%','%')
 
     def _iter_indented_subactions(self, action):
         try:

Lib/test/test_argparse.py

@@ -3934,6 +3934,39 @@ class TestHelpUsageWithParentheses(HelpTestCase):
         '''
     version = ''
 
+class TestHelpWithPercentageSymbols(HelpTestCase):
+    """Test a help message including % symbols"""
+    parser_signature = Sig(prog='PROG', description='Just a test code.')
+    argument_signatures = [
+        Sig('--somearg', metavar='somearg',
+            help='Now you dont need to escape this: %, and you will not get nonsensical errors!'),
+        Sig('--date', metavar='when',  help='A date in format %Y-%m-%d',
+            dest=f'date', type=str, required=True),
+        Sig('bar', nargs='?', type=int, default=42,
+            help='the bar to %(prog)s (default: %(default)s)'),
+        Sig('--weirdarg', metavar='weird', dest=f'weird', type=str,
+            required=True, help='A weird arg with 1 %, 2 %%% and 3 %%%%%'),
+    ]
+
+    usage = '''\
+        usage: PROG [-h] [--somearg somearg] --date when --weirdarg weird [bar]
+        '''
+    help = usage + '''\
+
+        Just a test code.
+
+        positional arguments:
+          bar                the bar to PROG (default: 42)
+
+        options:
+          -h, --help         show this help message and exit
+          --somearg somearg  Now you dont need to escape this: %, and you will not get
+                             nonsensical errors!
+          --date when        A date in format %Y-%m-%d
+          --weirdarg weird   A weird arg with 1 %, 2 %% and 3 %%%
+        '''
+    version = ''
+
 
 class TestHelpOnlyUserGroups(HelpTestCase):
     """Test basic usage messages"""

Misc/NEWS.d/next/Library/2023-08-23-01-09-48.gh-issue-44986.euCjji.rst

@@ -0,0 +1,6 @@
+Added feature to argparse so that percentage symbols (%) don't have to be
+escaped in help messages. Without this fix, they have to be escaped (as
+stated in the documentation) but if you forget this, the error message that
+is produced is completely cryptic. With this patch, 1) this does not happen
+anymore, 2) single % don't need to be escaped and 3) it remains fully
+compatible with the previous implementation.