_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)