Index: Lib/idlelib/IOBinding.py
===================================================================
--- Lib/idlelib/IOBinding.py	(revision 72282)
+++ Lib/idlelib/IOBinding.py	(working copy)
@@ -525,6 +525,7 @@
         ("All text files", "*", "TEXT"),
         ("All files", "*"),
         ]
+    defaultextension = ".py"
 
     def askopenfile(self):
         dir, base = self.defaultfilename("open")
@@ -551,8 +552,10 @@
     def asksavefile(self):
         dir, base = self.defaultfilename("save")
         if not self.savedialog:
-            self.savedialog = tkFileDialog.SaveAs(master=self.text,
-                                                  filetypes=self.filetypes)
+            self.savedialog = tkFileDialog.SaveAs(
+                master=self.text,
+                filetypes=self.filetypes,
+                defaultextension=self.defaultextension)
         filename = self.savedialog.show(initialdir=dir, initialfile=base)
         if isinstance(filename, unicode):
             filename = filename.encode(filesystemencoding)