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 @@ def test_join(self):
         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):

Objects/stringobject.c

@@ -1618,22 +1618,18 @@ string_join(PyStringObject *self, PyObject *orig)
 	}
 	if (seqlen == 1) {
 		item = PySequence_Fast_GET_ITEM(seq, 0);
-		if (!PyString_Check(item) && !PyUnicode_Check(item)) {
-			PyErr_Format(PyExc_TypeError,
-				     "sequence item 0: expected string,"
-				     " %.80s found",
-				     item->ob_type->tp_name);
+		if (PyString_CheckExact(item) || PyUnicode_CheckExact(item)) {
+			Py_INCREF(item);
 			Py_DECREF(seq);
-			return NULL;
+			return item;
 		}
-		Py_INCREF(item);
-		Py_DECREF(seq);
-		return item;
 	}
 
-	/* There are at least two things to join.  Do a pre-pass to figure out
-	 * the total amount of space we'll need (sz), see whether any argument
-	 * is absurd, and defer to the Unicode join if appropriate.
+	/* There are at least two things to join, or else we have a subclass
+	 * of the builtin types in the sequence.  
+	 * Do a pre-pass to figure out the total amount of space we'll
+	 * need (sz), see whether any argument is absurd, and defer to
+	 * the Unicode join if appropriate.
 	 */
 	for (i = 0; i < seqlen; i++) {
 		const size_t old_sz = sz;