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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):
 """ Name if the MOP formatted file to parse. @rtype: basestring @return: filename """

        if self._filename is None:
            self._filename = storage.get_file(self.basename,
                                              self.ccd,
                                              ext=self.extension,
                                              version=self.type,
                                              prefix=self.prefix)
        return self._filename