def _fixGSA(self):
        """
        Extracts the information regarding which satellites were used in
        obtaining the GPS fix from a GSA sentence.

        Precondition: A GSA sentence was fired. Postcondition: The current
        sentence data (C{self._sentenceData} will contain a set of the
        currently used PRNs (under the key C{_usedPRNs}.
        """
        self._sentenceData['_usedPRNs'] = set()
        for key in ("usedSatellitePRN_%d" % (x,) for x in range(12)):
            prn = getattr(self.currentSentence, key, None)
            if prn is not None:
                self._sentenceData['_usedPRNs'].add(int(prn))

        Extracts the information regarding which satellites were used in
        obtaining the GPS fix from a GSA sentence.

        Precondition: A GSA sentence was fired. Postcondition: The current
        sentence data (C{self._sentenceData} will contain a set of the
        currently used PRNs (under the key C{_usedPRNs}.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/positioning/nmea.py