gh-148468: Accept string-like help proxies in argparse

Lib/argparse.py

@@ -758,7 +758,7 @@ def colorize(match):
             return spec % params
 
         return self._apply_text_markup(
-            _re.sub(fmt_spec, colorize, help_string, flags=_re.VERBOSE)
+            _re.sub(fmt_spec, colorize, str(help_string), flags=_re.VERBOSE)
         )
 
     def _iter_indented_subactions(self, action):

Lib/test/test_argparse.py

@@ -7837,6 +7837,38 @@ def test_backtick_markup_in_argument_help_color_disabled(self):
         self.assertIn("set the `foo` value", help_text)
         self.assertNotIn("\x1b[", help_text)
 
+    def test_argument_help_interpolation_accepts_string_like_proxy(self):
+        class LazyStr:
+            def __init__(self, message):
+                self._message = message
+
+            def __str__(self):
+                return self._message
+
+            def __contains__(self, item):
+                return item in self._message
+
+            def __mod__(self, other):
+                return self._message % other
+
+            def strip(self):
+                return self._message.strip()
+
+        parser = argparse.ArgumentParser(prog="PROG", color=True)
+        parser.add_argument(
+            "--foo",
+            default="bar",
+            help=LazyStr("set `foo` (default: %(default)s)"),
+        )
+
+        prog_extra = self.theme.prog_extra
+        interp = self.theme.interpolated_value
+        reset = self.theme.reset
+
+        help_text = parser.format_help()
+        self.assertIn(f"set {prog_extra}foo{reset}", help_text)
+        self.assertIn(f"default: {interp}bar{reset}", help_text)
+
     def test_help_with_format_specifiers(self):
         # GH-142950: format specifiers like %x should work with color=True
         parser = argparse.ArgumentParser(prog='PROG', color=True)

Misc/NEWS.d/next/Library/2026-05-22-17-53-01.gh-issue-148468.o5fZ3K.rst

@@ -0,0 +1,3 @@
+Fix a regression in :mod:`argparse` so colorized help formatting once again
+accepts string-like proxy objects for descriptions, epilogs, and argument
+help text.