def apply_discounts(prices, applied=[]):

    total = 0

    for p in prices:

        discounted = round(p * 0.9, 2)

        applied.append(discounted)

        total += discounted

    print(f"Applied {len(applied)} discounts so far")

    return total

cart_a = apply_discounts([100.0, 50.0])

cart_b = apply_discounts([20.0])

print(cart_b, "->", len(apply_discounts.__defaults__[0]))