Python中的不支持操作数类型

Question

我写了一个非常简单的脚本，用来获取购买的产品数量、成本和平均值：

from __future__ import division

def print_purchase(arg1, arg2, arg3):
    print """You bought %r products and paid %r,
             for an average of %d""" % (arg1, arg2, arg3)

quantity = raw_input("How many products did you buy?")
cost = raw_input("How much did you pay?")
average = quantity/cost

print_purchase(quantity, cost, average)

这个脚本运行得很好，直到需要进行除法运算的时候。我尝试了几种方法来“修改”代码，以便它可以进行这些运算（比如导入除法功能等等），但我还是没能让它正常工作：

Traceback (most recent call last):
  File "purchase.py", line 9, in <module>
    average = quantity/cost
TypeError: unsupported operand type(s) for /: 'str' and 'str'

Answer 1

函数 raw_input() 会把你输入的内容当作一个 字符串 返回，所以你需要把它转换成数字（可以是 int 整数或者 float 浮点数）：

quantity = int(raw_input("How many products did you buy?"))
cost = float(raw_input("How much did you pay?"))

Answer 2

你应该把输入的文本类型（string）转换成数字类型（float 或 int）；

from __future__ import division

def print_purchase(arg1, arg2, arg3):
    print """You bought %r products and paid %r,
             for an average of %d""" % (arg1, arg2, arg3)
try:
    quantity = float(raw_input("How many products did you buy?"))
    cost = float(raw_input("How much did you pay?"))
except (TypeError, ValueError): 
    print ("Not numeric. Try Again.")      

print_purchase(quantity, cost, average) 
average = quantity/cost