Getting wrong answer

I would like to give some feedback about ...

From: https://py.checkio.org/mission/army-battles/solve/

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Below is my code. For some reason it says i am getting the wrong answer for the test:

army_1 = Army()

army_2 = Army()

army1.addunits(Warrior, 20)

army2.addunits(Warrior, 21)

battle = Battle()

battle.fight(army1, army2)

I get false, but it says it should be true. I don't see how this should be the case.

class Warrior:
    def __init__(self):
        self.health = 50
        self.attack = 5
        self.is_alive = True

class Knight(Warrior):
    def __init__(self):
        super().__init__()
        self.attack = 7

class Army:
    def __init__(self):
        self.soldiers = []

    def add_units(self,soldier, amt):
        for _ in range(amt):
            self.soldiers.append(soldier())

class Battle:
    def __init__(self):
        pass

    def fight(self,a1,a2):
        while len(a1.soldiers) > 0 and len(a2.soldiers) > 0:
            soldier_alive = fight(a1.soldiers[0],a2.soldiers[0])
            if soldier_alive:
                a2.soldiers.pop(0)
            else:
                a1.soldiers.pop(0)

        if len(a1.soldiers) > 0:
            return True
        else:
            return False

def fight(unit_1, unit_2):
    while unit_1.is_alive and unit_2.is_alive:
        unit_2.health -= unit_1.attack
        if unit_2.is_alive:
            unit_1.health -= unit_2.attack

        if unit_2.health <= 0:
            unit_2.is_alive = False
            return True

        if unit_1.health <= 0:
            unit_1.is_alive = False
            return False


if __name__ == '__main__':
    #These "asserts" using only for self-checking and not necessary for auto-testing

    #fight tests
    chuck = Warrior()
    bruce = Warrior()
    carl = Knight()
    dave = Warrior()
    mark = Warrior()

    assert fight(chuck, bruce) == True
    assert fight(dave, carl) == False
    assert chuck.is_alive == True
    assert bruce.is_alive == False
    assert carl.is_alive == True
    assert dave.is_alive == False
    assert fight(carl, mark) == False
    assert carl.is_alive == False

    #battle tests
    my_army = Army()
    my_army.add_units(Knight, 3)

    enemy_army = Army()
    enemy_army.add_units(Warrior, 3)

    army_3 = Army()
    army_3.add_units(Warrior, 20)
    army_3.add_units(Knight, 5)

    army_4 = Army()
    army_4.add_units(Warrior, 30)

    battle = Battle()

    assert battle.fight(my_army, enemy_army) == True
    assert battle.fight(army_3, army_4) == False
    print("Coding complete? Let's try tests!")