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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 apcor(self):
 """ return the aperture correction of for the CCD assocated with the reading. @return: Apcor """

        if self._apcor is None:
            try:
                self._apcor = Downloader().download_apcor(self.reading.get_apcor_uri())
            except:
                self._apcor = ApcorData.from_string("5 15 99.99 99.99")
        return self._apcor