getLeader(string, double) -> (string, double)

        Return the leading vehicle id together with the distance. The distance
        is measured from the front + minGap to the back of the leader, so it does not include the
        minGap of the vehicle.
        The dist parameter defines the maximum lookahead, 0 calculates a lookahead from the brake gap.
        Note that the returned leader may be farther away than the given dist.

def getLeader(self, vehID, dist=0.):
        """getLeader(string, double) -> (string, double)

        Return the leading vehicle id together with the distance. The distance
        is measured from the front + minGap to the back of the leader, so it does not include the
        minGap of the vehicle.
        The dist parameter defines the maximum lookahead, 0 calculates a lookahead from the brake gap.
        Note that the returned leader may be farther away than the given dist.
        """
        self._connection._beginMessage(
            tc.CMD_GET_VEHICLE_VARIABLE, tc.VAR_LEADER, vehID, 1 + 8)
        self._connection._string += struct.pack("!Bd", tc.TYPE_DOUBLE, dist)
        return _readLeader(self._connection._checkResult(tc.CMD_GET_VEHICLE_VARIABLE, tc.VAR_LEADER, vehID))