diff -r e8d89503f061 Lib/idlelib/IOBinding.py
--- a/Lib/idlelib/IOBinding.py	Sat May 26 20:44:45 2012 -0400
+++ b/Lib/idlelib/IOBinding.py	Sun May 27 14:07:40 2012 -0500
@@ -156,7 +156,7 @@
                 self.filename_change_hook()
 
     def open(self, event=None, editFile=None):
-        flist = self.editwin.flist
+        flist = self.editwin.flist  # Save flist in case parent window closes.
         if flist:
             if not editFile:
                 filename = self.askopenfile()
@@ -164,23 +164,24 @@
                 filename=editFile
             if filename:
                 # If the current window has no filename and hasn't been
-                # modified, we replace its contents (no loss).  Otherwise
-                # we open a new window.  But we won't replace the
-                # shell window (which has an interp(reter) attribute), which
-                # gets set to "not modified" at every new prompt.
-                # Also, make sure the current window has not been closed,
-                # since it can be closed during the Open File dialog.
-                try:
-                    interp = self.editwin.interp
-                except AttributeError:
-                    interp = None
-
-                if self.editwin and not self.filename and \
-                          self.get_saved() and not interp:
+                # modified, we replace its contents (no loss). Otherwise
+                # we open a new window. A shell window (which has an
+                # interp(reter) attribute, no filename, and gets set to
+                # "not modified" at every new prompt) will never have
+                # its contents replaced.
+                # Also, if the current window was closed during askopenfile,
+                # then open in a new window.
+                if (self.editwin and
+                        not getattr(self.editwin, 'interp', None) and
+                        not self.filename and
+                        self.get_saved()):
+                    # Open into the blank, unmodified Editor Window
                     flist.open(filename, self.loadfile)
                 else:
+                    # Open in a new Editor Window
                     flist.open(filename)
             else:
+                # No file to open
                 if self.text:
                     self.text.focus_set()