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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 zmag(self,):
 """ Return the photometric zeropoint of the CCD associated with the reading. @return: float """

        if self._zmag is None:
            hdulist_index = self.get_hdulist_idx(self.reading.get_ccd_num())
            self._zmag = self.hdulist[hdulist_index].header.get('PHOTZP', 0.0)
        return self._zmag