is.alive problem

Hey, guys!

Please help me find where is a problem. I stopped at fight no. 5. My code set wife.is_alive = True, but it should be False. Here is my code:

class Warrior:
    def __init__(self):
        self.health = 50
        self.attack = 5
        self.health_list = [50]
        self.is_alive = (self.health_list[-1] >= 0)

class Knight(Warrior):
    def __init__(self):
        Warrior.__init__(self)
        self.health = 50
        self.attack = 7
        self.health_list = [50]
        self.is_alive = (self.health_list[-1] >= 0)


def fight(unit_1, unit_2):
    r = 2
    while unit_1.health > 0 and unit_2.health > 0:

        if r % 2 == 0: #round 1
            unit_2.health -= unit_1.attack
            unit_2.health_list.append(unit_2.health)
            r += 1

        else:  #round 2
            unit_1.health -= unit_2.attack
            unit_1.health_list.append(unit_1.health)
            r += 1

    if (unit_1.health > 1):
        return True
    else:
        return False