Index: Lib/idlelib/IOBinding.py
===================================================================
--- Lib/idlelib/IOBinding.py	(Revision 68173)
+++ Lib/idlelib/IOBinding.py	(Arbeitskopie)
@@ -7,7 +7,7 @@
 import tkinter.messagebox as tkMessageBox
 import re
 from tkinter import *
-from tkinter.simpledialog import SimpleDialog
+from tkinter.simpledialog import askstring
 
 from idlelib.configHandler import idleConf
 
@@ -281,8 +281,19 @@
         # Finally, try the locale's encoding. This is deprecated;
         # the user should declare a non-ASCII encoding
         try:
-            chars = str(bytes, locale_encoding)
-            self.fileencoding = locale_encoding
+            # Wait for the editor window to appear
+            self.editwin.text.update()
+            enc = askstring(
+                "Specify file encoding",
+                "The file's encoding is invalid for Python 3.x.\n"
+                "IDLE will convert it to UTF-8.\n"
+                "What is the current encoding of the file?",
+                initialvalue = locale_encoding,
+                parent = self.editwin.text)
+
+            if enc:
+                chars = str(bytes, enc)
+                self.fileencoding = None
         except UnicodeDecodeError:
             pass
         return chars  # None on failure