Index: Lib/idlelib/IOBinding.py
===================================================================
--- Lib/idlelib/IOBinding.py	(revision 74276)
+++ Lib/idlelib/IOBinding.py	(working copy)
@@ -526,6 +526,8 @@
         ("All files", "*"),
         ]
 
+    defaultextension = '.py' if sys.platform[:3] == 'win' else ''
+
     def askopenfile(self):
         dir, base = self.defaultfilename("open")
         if not self.opendialog:
@@ -551,8 +553,10 @@
     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)