class test(object):
    def __init__(self, x=None, y=None, z=None):
        pass

class mytest(test):
    def __init__(self, x=None, y=None, z=None):
        self._x = x
        self._y = y
        self._z = z
        super().__init__(x, y, z)

    def get_x(self):
        return self._x
    # and so on