input函数出现的问题

input函数出现的问题(Python)

参考书<A Learner's Guide to Programming Using the Python Language>,写下如下的程序:

 # coding=utf-8

 # 以上是为了能在文中添加中文的注释

 def save_transaction(price, credit_card, description):

     file = open("transaction.txt", "a")

     file.write("%s%07d%16s\n" % (credit_card, price, description))

     file.close()

 items = ["DONUT","LATTE","FILTER","MUFFIN"]

 prices = [1.50, 2.0, 1.80, 1.20]

 running = True

 while running:

     option = 1

     for choice in items:

         print(str(option) + "." + choice)

         option = option + 1

     print(str(option) + ".Quit")

     choice = int(input("Choose an option: "))

     if option == choice:

         running = False

     else:

         #用input的话，如果是以0开始的字符串的话，就会报错

         credit_card = input("Credit card number: ")

         while len(credit_card) != 16 :

             credit_card = input("the length of credit card number must be equal to 16. Please input Credit card number again: ")

         save_transaction(prices[choice-1]*100, credit_card, items[choice-1])

但是，在运行到第26行的时候，如果输入“0987648900804387”时，会出现如下的错误:

Traceback (most recent call last):

  File "transaction.py", line 26, in <module>

    credit_card = input("Credit card number: ")

  File "<string>", line 1987648900804387

                  ^

SyntaxError: invalid token

上网查了下，注意到一个细节：

input其实是通过raw_input来实现的，原理很简单，就下面一行代码:[参考 http://www.cnblogs.com/linlu11/archive/2009/11/25/1610830.html]

def input(prompt):

    return (eval(raw_input(prompt)))

那么，简单写了测试函数:

>>> eval("")

83

>>> eval("")

Traceback (most recent call last):

  File "<stdin>", line 1, in <module>

  File "<string>", line 1987

      ^

SyntaxError: invalid token

>>> eval("0x9d")

157

>>> eval("0x9h")

Traceback (most recent call last):

  File "<stdin>", line 1, in <module>

  File "<string>", line 1

    0x9h

      ^

SyntaxError: unexpected EOF while parsing

可见，在input函数中，调用eval时候，会根据字符串中的第一个或者前2个字符来判断字符串对应是8进制还是16进制的，

如果是0，就是按照8进制来处理：所以上面的输入是 0987648900804387 非法的,不是一个有效的8进制数;

如果是0x，就按照16进制来处理，所以0x9h也是非法的。

因此，原程序中的第26行到28行，可以换成：

         credit_card = raw_input("Credit card number: ")

         while len(credit_card) != 16 :

             credit_card = raw_input("the length of credit card number must be equal to 16. Please input Credit card number again: ")