class Topping:
def __init__(self, value):
self._value = value
self._pizza = []

def __repr__(self):
return 'Topping [{!r}]'.format(self._value)

def add_topping(self, topping):
self._pizza.append(topping)

def __iter__(self):
return iter(self._pizza)

if __name__ == '__main__':
base = Topping('dough')

topping1 = Topping('cheese')
topping2 = Topping('shrimp')
topping3 = Topping('green pepper')
topping4 = Topping(4)

base.add_topping(topping1)
base.add_topping(topping2)
base.add_topping(topping3)
base.add_topping(topping4)

for tp in base:
print(tp)

# Topping ['cheese']
# Topping ['shrimp']
# Topping ['green pepper']
# Topping [4]