Index: Lib/tempfile.py
===================================================================
--- Lib/tempfile.py	(revision 85026)
+++ Lib/tempfile.py	(working copy)
@@ -596,8 +596,13 @@
     def tell(self):
         return self._file.tell()
 
-    def truncate(self):
-        self._file.truncate()
+    def truncate(self,size=None):
+        if size is None:
+            self._file.truncate()
+        else:
+            if size > self._max_size:
+                self.rollover()
+            self._file.truncate(size)
 
     def write(self, s):
         file = self._file
Index: Lib/test/test_tempfile.py
===================================================================
--- Lib/test/test_tempfile.py	(revision 85026)
+++ Lib/test/test_tempfile.py	(working copy)
@@ -828,7 +828,29 @@
                 pass
         self.assertRaises(ValueError, use_closed)
 
+    def test_truncate_with_size_parameter(self):
+        # A SpooledTemporaryFile can be truncated to zero size
+        f = tempfile.SpooledTemporaryFile(max_size=10)
+        f.write(b'abcdefg\n')
+        f.seek(0)
+        f.truncate()
+        self.assertFalse(f._rolled)
+        self.assertEquals(f._file.getvalue(),b'')
+        # A SpooledTemporaryFile can be truncated to a specific size
+        f = tempfile.SpooledTemporaryFile(max_size=10)
+        f.write(b'abcdefg\n')
+        f.truncate(4)
+        self.assertFalse(f._rolled)
+        self.assertEquals(f._file.getvalue(),b'abcd')
+        # A SpooledTemporaryFile rolls over if truncated to large size
+        f = tempfile.SpooledTemporaryFile(max_size=10)
+        f.write(b'abcdefg\n')
+        f.truncate(20)
+        self.assertTrue(f._rolled)
+        if has_stat:
+            self.assertEquals(os.fstat(f.fileno()).st_size,20)
 
+
 test_classes.append(test_SpooledTemporaryFile)