gh-159868: Accept UserString arguments in startswith and endswith

Lib/collections/__init__.py

@@ -1536,6 +1536,8 @@ def encode(self, encoding='utf-8', errors='strict'):
         return self.data.encode(encoding, errors)
 
     def endswith(self, suffix, start=0, end=_sys.maxsize):
+        if isinstance(suffix, UserString):
+            suffix = suffix.data
         return self.data.endswith(suffix, start, end)
 
     def expandtabs(self, tabsize=8):
@@ -1646,6 +1648,8 @@ def splitlines(self, keepends=False):
         return self.data.splitlines(keepends)
 
     def startswith(self, prefix, start=0, end=_sys.maxsize):
+        if isinstance(prefix, UserString):
+            prefix = prefix.data
         return self.data.startswith(prefix, start, end)
 
     def strip(self, chars=None):

Lib/test/test_userstring.py

@@ -121,6 +121,32 @@ def test_mixed_cmp(self):
         self._assert_cmp(a, 'a', 1)
         self._assert_cmp(a, 'ac', -1)
 
+    @support.subTests('str_class', (UserString, UserStringSubclass))
+    def test_startswith_userstring(self, str_class):
+        s = UserString('hello world')
+        self.assertTrue(s.startswith(str_class('hello')))
+        self.assertFalse(s.startswith(str_class('world')))
+        self.assertTrue(s.startswith(str_class('')))
+        self.assertFalse(s.startswith(str_class('hello world!')))
+        self.assertTrue(s.startswith(str_class('world'), 6))
+        self.assertTrue(s.startswith(str_class('world'), 6, 11))
+        self.assertFalse(s.startswith(str_class('world'), 6, 10))
+        self.assertTrue(s.startswith(str_class('world'), -5))
+        self.assertTrue(s.startswith(str_class('world'), start=6, end=11))
+
+    @support.subTests('str_class', (UserString, UserStringSubclass))
+    def test_endswith_userstring(self, str_class):
+        s = UserString('hello world')
+        self.assertTrue(s.endswith(str_class('world')))
+        self.assertFalse(s.endswith(str_class('hello')))
+        self.assertTrue(s.endswith(str_class('')))
+        self.assertFalse(s.endswith(str_class('hello world!')))
+        self.assertTrue(s.endswith(str_class('world'), 6))
+        self.assertTrue(s.endswith(str_class('hello'), 0, 5))
+        self.assertFalse(s.endswith(str_class('hello'), 1, 5))
+        self.assertTrue(s.endswith(str_class('hello'), -11, -6))
+        self.assertTrue(s.endswith(str_class('hello'), start=0, end=5))
+
     def test_rmod(self):
         class ustr2(UserString):
             pass

Misc/NEWS.d/next/Library/2026-10-20-16-00-00.gh-issue-159868.qLhYkP.rst

@@ -0,0 +1,3 @@
+Allow :meth:`!collections.UserString.startswith` and
+:meth:`!collections.UserString.endswith` to accept a
+:class:`~collections.UserString` instance as the prefix or suffix argument.