1. 检查特殊元素
在for语句中嵌套if语句：
request_toppings = ['mushrooms','green peppers','extra cheese']
for request_topping in request_toppings: #遍历其中每一个元素
if(request_topping == 'green peppers'): #判断遍历过的元素是否有等于green peppers的
print("Sorry, we are run out of green peppers")
else:
print("Adding " + request_topping + '.')
print("\nFinished your pizza!")
2. 检查空元素
request_toppings = []
if request_toppings: #判断是否是空元素，如果不是空，执行跟上面相同的步骤
for request_topping in request_toppings:
print("Adding " + request_topping)
print("\nFinished your pizza!")
else: #如果是空元素有如下输出
print("Are you sure you want a plain pizza?")
3. 多个列表
available_toppings = ['mushrooms','olives','green peppers','pepperoni','pineapple','extra cheese']
request_toppings = ['mushrooms','french fries','extra cheese']
for request_topping in request_toppings: #遍历第二个列表中所有元素
if request_topping in available_toppings: #判断是否第二个列表中的所有元素都在第一个列表中
print("Adding " + request_topping + '.')
else:
print("Sorry we donot have " + request_topping + '.')
print("\nFinished making your pizza!")
