Python - ShoppingCart & Shop

class ShoppingCart:

    def __init__ (self):
        self.total = 0
        self.items = {}

    def add_item(self, item_name, quantity, price):
        self.total += quantity * price
        self.items[item_name] = quantity

    def remove_item(self, item_name, quantity, price):
        if quantity > self.items[item_name]:
            quantity = self.items[item_name]

        self.total -= quantity * price
        self.items[item_name] -= quantity

    def checkout(self, cash_paid):
        if cash_paid < self.total:
            return "Cash paid not enough"

        return cash_paid - self.total

class Shop(ShoppingCart):

    def __init__(self):
        # ShoppingCart "constructor"
        super().__init__()

        self.quantity = 100

    def remove_item(self, item_name=None, quantity=None, price=None):
        if item_name is None or quantity is None or price is None:
            self.quantity -= 1
        else:
            # ShoppingCart "remove_item"
            super().remove_item(item_name, quantity, price)

# --- test ---

a = Shop()

print('\n--- adding items ---\n')

a.add_item('banana', 5, 4)
a.add_item('orange', 3, 5)

print('total:', a.total)
print('items:', a.items)
print('quantity:', a.quantity)

print('\n--- removing without arguments ---\n')

a.remove_item()

print('total:', a.total)
print('items:', a.items)
print('quantity:', a.quantity)

print('\n--- removing 6 bananas ---\n')

a.remove_item('banana', 6, 4)

print('total:', a.total)
print('items:', a.items)
print('quantity:', a.quantity)

print('\n--- removing 3 bananas ---\n')

a.remove_item('banana', 3, 4)

print('total:', a.total)
print('items:', a.items)
print('quantity:', a.quantity)

print('\n--- checkout ---\n')

print('result:', a.checkout(10))
print('result:', a.checkout(50))

'''
--- adding items ---

total: 35
items: {'banana': 5, 'orange': 3}
quantity: 100

--- removing without arguments ---

total: 35
items: {'banana': 5, 'orange': 3}
quantity: 99

--- removing 6 bananas ---

total: 15
items: {'banana': 0, 'orange': 3}
quantity: 99

--- removing 3 bananas ---

total: 15
items: {'banana': 0, 'orange': 3}
quantity: 99

--- checkout ---

result: Cash paid not enough
result: 35
'''