SF Patch #1007087: Return new string for single subclass joins (Bug #1001011)

(Patch contributed by Nick Coghlan.)

Now joining string subtypes will always return a string.
Formerly, if there were only one item, it was returned unchanged.

Lib/test/test_string.py

@@ -52,6 +52,29 @@ class StringTest(
         self.checkraises(TypeError, string_tests.BadSeq1(), 'join', ' ')
         self.checkequal('a b c', string_tests.BadSeq2(), 'join', ' ')
 
+    def test_bug1001011(self):
+        # Make sure join returns a NEW object for single item sequences
+        # involving a subclass
+        # Make sure that it is of the appropriate type
+        # Check the optimisation still occurs for standard objects
+        class str_subclass(str): pass
+        s1 = str_subclass('abcd')
+        s2 = ''.join([s1])
+        self.failIf(s1 is s2)
+        self.assertEqual(type(s2), type(''))
+        s3 = 'abcd'
+        s4 = ''.join([s3])
+        self.failUnless(s3 is s4)
+        if test_support.have_unicode:
+            class unicode_subclass(unicode): pass
+            u1 = unicode_subclass(u'abcd')
+            u2 = ''.join([u1])
+            self.failIf(u1 is u2)
+            self.assertEqual(type(u2), type(u''))
+            u3 = u'abcd'
+            u4 = ''.join([u3])
+            self.failUnless(u3 is u4)
+
 class ModuleTest(unittest.TestCase):
 
     def test_attrs(self):