Add support for negative indices in UserString.MutableString.__setitem__

and UserString.MutableString.__delitem__.

Lib/UserString.py

@@ -146,9 +146,13 @@ class MutableString(UserString):
     def __hash__(self):
         raise TypeError, "unhashable type (it is mutable)"
     def __setitem__(self, index, sub):
+        if index < 0:
+           index += len(self.data)
         if index < 0 or index >= len(self.data): raise IndexError
         self.data = self.data[:index] + sub + self.data[index+1:]
     def __delitem__(self, index):
+        if index < 0:
+           index += len(self.data)
         if index < 0 or index >= len(self.data): raise IndexError
         self.data = self.data[:index] + self.data[index+1:]
     def __setslice__(self, start, end, sub):

Lib/test/test_userstring.py

@@ -52,20 +52,20 @@ class MutableStringTest(UserStringTest):
 
     def test_setitem(self):
         s = self.type2test("foo")
-        self.assertRaises(IndexError, s.__setitem__, -1, "bar")
+        self.assertRaises(IndexError, s.__setitem__, -4, "bar")
         self.assertRaises(IndexError, s.__setitem__, 3, "bar")
+        s[-1] = "bar"
+        self.assertEqual(s, "fobar")
         s[0] = "bar"
-        self.assertEqual(s, "baroo")
-        s[4] = "foo"
-        self.assertEqual(s, "barofoo")
+        self.assertEqual(s, "barobar")
 
     def test_delitem(self):
         s = self.type2test("foo")
-        self.assertRaises(IndexError, s.__delitem__, -1)
+        self.assertRaises(IndexError, s.__delitem__, -4)
         self.assertRaises(IndexError, s.__delitem__, 3)
+        del s[-1]
+        self.assertEqual(s, "fo")
         del s[0]
-        self.assertEqual(s, "oo")
-        del s[1]
         self.assertEqual(s, "o")
         del s[0]
         self.assertEqual(s, "")

Misc/NEWS

@@ -152,6 +152,9 @@ Library
 - The reconvert.quote function can now emit triple-quoted strings.  The
   reconvert module now has some simple documentation.
 
+- ``UserString.MutableString`` now supports negative indices in
+  ``__setitem__`` and ``__delitem__``
+
 Build
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