def _getDOP(self, dopType):
        """
        Gets a particular dilution of position value.

        @param dopType: The type of dilution of position to get. One of
            ('pdop', 'hdop', 'vdop').
        @type dopType: C{str}
        @return: The DOP if it is known, L{None} otherwise.
        @rtype: C{float} or L{None}
        """
        for dopExpression in self._DOP_EXPRESSIONS[dopType]:
            try:
                return dopExpression(self)
            except TypeError:
                continue

        Gets a particular dilution of position value.

        @param dopType: The type of dilution of position to get. One of
            ('pdop', 'hdop', 'vdop').
        @type dopType: C{str}
        @return: The DOP if it is known, L{None} otherwise.
        @rtype: C{float} or L{None}
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/positioning/base.py