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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:

def _filename(self_or_cls, filename):
        "Add the file extension if not already present"
        if not filename.endswith(self_or_cls.file_ext):
            return '%s.%s' % (filename, self_or_cls.file_ext)
        else:
            return filename