Issue 10365: Add and replace comments; condense defaulted attribute access.

Code patch by Roger Serwy.

Lib/idlelib/IOBinding.py

@@ -157,35 +157,32 @@ class IOBinding:
 
     def open(self, event=None, editFile=None):
         flist = self.editwin.flist
+        # Save in case parent window is closed (ie, during askopenfile()).
         if flist:
             if not editFile:
                 filename = self.askopenfile()
             else:
                 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:
+                # If editFile is valid and already open, flist.open will
+                # shift focus to its existing window.
+                # If the current window exists and is a fresh unnamed,
+                # unmodified editor window (not an interpreter shell),
+                # pass self.loadfile to flist.open so it will load the file
+                # in the current window (if the file is not already open)
+                # instead of a new window.
+                if (self.editwin and
+                        not getattr(self.editwin, 'interp', None) and
+                        not self.filename and
+                        self.get_saved()):
                     flist.open(filename, self.loadfile)
                 else:
                     flist.open(filename)
             else:
                 if self.text:
                     self.text.focus_set()
-
             return "break"
-        #
+
         # Code for use outside IDLE:
         if self.get_saved():
             reply = self.maybesave()