_orig_open = open

class _BkFile:
   def __init__(self, file, mode, bufsize):
       import os
       self.__filename = file
       self.__backup = file + '~'
       try:
           os.unlink(self.__backup)
       except os.error:
           pass
       try:
           os.rename(file, self.__backup)
       except os.error:
           self.__backup = None
       self.__file = _orig_open(file, mode, bufsize)
       self.closed = self.__file.closed
       self.fileno = self.__file.fileno
       self.flush = self.__file.flush
       self.isatty = self.__file.isatty
       self.mode = self.__file.mode
       self.name = self.__file.name
       self.read = self.__file.read
       self.readinto = self.__file.readinto
       self.readline = self.__file.readline
       self.readlines = self.__file.readlines
       self.seek = self.__file.seek
       self.softspace = self.__file.softspace
       self.tell = self.__file.tell
       self.truncate = self.__file.truncate
       self.write = self.__file.write
       self.writelines = self.__file.writelines

   def close(self):
       self.__file.close()
       if self.__backup is None:
           return
       import filecmp
       if filecmp.cmp(self.__backup, self.__filename, shallow = 0):
           import os
           os.unlink(self.__filename)
           os.rename(self.__backup, self.__filename)

def open(file, mode = 'r', bufsize = -1):
   if 'w' not in mode:
       return _orig_open(file, mode, bufsize)
   return _BkFile(file, mode, bufsize)

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