gh-140911: Ensure that UserString.rindex(), UserString.index() accept UserString as sub argument.

Lib/collections/__init__.py

@@ -1542,6 +1542,8 @@ def format_map(self, mapping):
         return self.data.format_map(mapping)
 
     def index(self, sub, start=0, end=_sys.maxsize):
+        if isinstance(sub, UserString):
+            sub = sub.data
         return self.data.index(sub, start, end)
 
     def isalpha(self):
@@ -1610,6 +1612,8 @@ def rfind(self, sub, start=0, end=_sys.maxsize):
         return self.data.rfind(sub, start, end)
 
     def rindex(self, sub, start=0, end=_sys.maxsize):
+        if isinstance(sub, UserString):
+            sub = sub.data
         return self.data.rindex(sub, start, end)
 
     def rjust(self, width, *args):

Lib/test/string_tests.py

@@ -295,6 +295,32 @@ def test_index(self):
         else:
             self.checkraises(TypeError, 'hello', 'index', 42)
 
+        # For a variety of combinations,
+        #    verify that str.index() matches __contains__
+        #    and that the found substring is really at that location
+        charset = ['', 'a', 'b', 'c']
+        digits = 5
+        base = len(charset)
+        teststrings = set()
+        for i in range(base ** digits):
+            entry = []
+            for j in range(digits):
+                i, m = divmod(i, base)
+                entry.append(charset[m])
+            teststrings.add(''.join(entry))
+        teststrings = [self.fixtype(ts) for ts in teststrings]
+        for i in teststrings:
+            for j in teststrings:
+                try:
+                    loc = i.index(j)
+                except ValueError:
+                    loc = -1
+                r1 = (loc != -1)
+                r2 = j in i
+                self.assertEqual(r1, r2)
+                if loc != -1:
+                    self.assertEqual(i[loc:loc+len(j)], j)
+
     def test_rindex(self):
         self.checkequal(12, 'abcdefghiabc', 'rindex', '')
         self.checkequal(3,  'abcdefghiabc', 'rindex', 'def')
@@ -321,6 +347,32 @@ def test_rindex(self):
         else:
             self.checkraises(TypeError, 'hello', 'rindex', 42)
 
+        # For a variety of combinations,
+        #    verify that str.rindex() matches __contains__
+        #    and that the found substring is really at that location
+        charset = ['', 'a', 'b', 'c']
+        digits = 5
+        base = len(charset)
+        teststrings = set()
+        for i in range(base ** digits):
+            entry = []
+            for j in range(digits):
+                i, m = divmod(i, base)
+                entry.append(charset[m])
+            teststrings.add(''.join(entry))
+        teststrings = [self.fixtype(ts) for ts in teststrings]
+        for i in teststrings:
+            for j in teststrings:
+                try:
+                    loc = i.rindex(j)
+                except ValueError:
+                    loc = -1
+                r1 = (loc != -1)
+                r2 = j in i
+                self.assertEqual(r1, r2)
+                if loc != -1:
+                    self.assertEqual(i[loc:loc+len(j)], j)
+
     def test_find_periodic_pattern(self):
         """Cover the special path for periodic patterns."""
         def reference_find(p, s):

Misc/NEWS.d/next/Library/2025-11-03-17-13-00.gh-issue-140911.7KFvSQ.rst

@@ -0,0 +1,3 @@
+:mod:`collections`: Ensure that the methods ``UserString.rindex()`` and
+``UserString.index()`` accept :class:`collections.UserString` instances as the
+sub argument.