Index: Lib/test/string_tests.py
===================================================================
--- Lib/test/string_tests.py	(revision 77312)
+++ Lib/test/string_tests.py	(working copy)
@@ -875,6 +875,10 @@
         self.checkequal(['abc', 'def', 'ghi', ''], "abc\ndef\r\nghi\n\r", 'splitlines')
         self.checkequal(['', 'abc', 'def', 'ghi', ''], "\nabc\ndef\r\nghi\n\r", 'splitlines')
         self.checkequal(['\n', 'abc\n', 'def\r\n', 'ghi\n', '\r'], "\nabc\ndef\r\nghi\n\r", 'splitlines', 1)
+        # Optimization may return the original line for immutable types
+        self.checkequal(['abc'], "abc", 'splitlines')
+        self.checkequal(['abc\n'], "abc\n", 'splitlines', True)
+        self.checkequal(['abc'], "abc\n", 'splitlines', False)
 
         self.checkraises(TypeError, 'abc', 'splitlines', 42, 42)