_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)