https://www.runoob.com/python/python-exceptions.html
1. 附自己学习成果：
#取四件商品价格总和减去最便宜的商品价格的数
import sys
def addlist():
try:
list.append(geterror(int(input())))
except ValueError:
print("错误！输入的值不为数字")
sys.exit()
def geterror(raw_input):
try:
if raw_input < 0:
raise ValueError
except ValueError:
print("错误！输入的值不为正数")
sys.exit()
else:
return(raw_input)
list = []
print("请分别输入四件商品价格")
print("第一件：")
addlist()
print("第二件：")
addlist()
print("第三件：")
addlist()
print("第四件：")
addlist()
Min = min(list)
Sum = 0
for i in range(len(list)):
Sum = Sum + list[i]
result = Sum - Min
print("总价为:",result)